I plan to visit this WHS late March (I know it's a gamble with the weather...)

Can someone provide tips about:
- Where to stay (I am considering Tavistock)
- Locations that have something to see indoors
- Worthwhile parts of this WHS in general

I will fly & drive in via Bristol, so probably will not make it to the far end of Cornwall.

I don't have recent practical experience of these sites so will bow to others who have, on any detail about the best "pay to enter" locations. On the whole in recent years you haven't been enjoying UK's "pay to enter" attractions/museums so I will forebear from recommending any!
a. Late March has the great advantage of being before the main holiday season when the roads and towns can get impossibly crowded. They even have special names for tourists down there (somewhat pejorative ones!) - "Emmets" in Cornwall and "Grockles" in Devon! Also a lot of towns and villages start their strictest parking regulations in April
b. It will be getting dark around 18.15
c. You must see some northern coastal parts of the site as the long southern internal valleys are pleasant but not comparable (Though I wouldn't be putting in a lot of effort to see the port of Hayle itself) - could just be St Agnes as not being quite so far from Bristol but the road round from St Ives past St Just through Zennor and on to Penzance is very fine (I wouldn't bother with Lands End though unless you have some reason for wanting to have been there.)
d. How important is it for you to have a single overnight location for 2 nights? Are you on the pm flight getting in c 16.45? My strategy would be to drive as far as possible/reasonable that evening e.g to Truro or Penzance (3hours) and then wend my way back slowly after doing the west Cornwall circuit with St Just/St Agnes on the Saturday and going back towards Bristol only as far (Plymouth area?) as you need to give you time to get to the Sunday fliight (c5pm?) and to see a couple of remaining Devon sites and the Tamar valley on the Sunday morning? Better than having to keep on going back to Tavistock? The sites do make a reasonable "line" rather than a couple of "circles"!
e. If you do go to Tavistock when going back east then continue over via Dartmoor and Princetown home of UKs most infamous Victorian prison! Unless of course it is misty in which case you wouldn't want to come across the Hound of the Baskervilles!

I'm just in the process of writting a review at the moment. We visited last May, and really enjoyed it, even if it isn't going to top many best WHS lists. Having a car is all but essential.

We stayed one night Truro, which was a a nice surprise, nothing spectacular but rather charming and some excellent food. The second night we stayed between Plymouth and Tavistock, handy for heading into southern Devon and Dorset.

We only had a quick stop in Tavistock, it seemed nice, but not especially remarkable. Dartmoor national park next to it was lovely though. We were heading right down to Lands End (don't bother going that far for the Land End fairground though) so wanted to be further down the peninsula.

The stand-out location we visited was St Agnes, which I think shows Cornwall at its best. There is a small mining museum, and a charming little beach with a surf life club, but the main mining sites are on the hills up above. This would be my must see from all the sites.

Also the open air meeting place at Gwenapp was unique and a worth making the detour to see. And Chalrestown was one of the more photogenic locations, and had some mining related museums.

St Just has access to the mines, if that is what you wanted. But if you've ruled out heading that far not relavant.

I wouldn't put too much hope in Redruth or Hayle being highlights, they may look good on paper but reality isn't so great.

Outside of the WHS. Zennor was very pleasant, we had an excellent Sunday roast in the pub there. We ended up skipping St Ives, we had planned to visit but access to the city centre is closed to non resident cars, and it felt very very busy. So we decided not to battle the crowds, March may be a better time though. If you fancy visiting Newquay, be prepared to encounter more British drinking parties than Amsterdam, Prague and Tallinn combined.

Bristol is worth a bit of time as well. The Docks and SS Great Britain, as well as Temple Meads Station were all previously on the T-list. There is also the second branch of Stanford's book shop. Their other store in London is perhaps my favourite shop in the world.

Thanks a lot both of you.

I will probably be going only for the weekend, so Saturday morning (9.45) until Sunday afternoon (5 pm). The best bet seems to drive directly to St. Agnes, visit that area, stay overnight in Truro, and visit the Tamar valley on Sunday.

If you only have time to head to the eastern parts Caradon Hill gives a good impression of the general industrial relics. It is interesting, but not spectacular.

Despite my disparaging remarks on Newquay, it is well geared up for tourists and will have plenty of accommodation choices, just be warned if you want a quiet meal on a Saturday night then ask for local advice on where the Stag and Hen parties head, and then go elsewhere.

As for Truro, we stayed at Manning's, it was rather pleasant, had a nice location and a good breakfast included. We also had a very good meal at Hooked.

The official site is also pretty good for showing what the general inscribed areas will be like.

In terms of sustenance Cornwall is very good; lots of seafood, clotted cream, pasties and cider.

Yes that is a bit tight isn't it. Bristol airport is on the right side of Bristol for you but fairly slow to/from the motorway. If you arrive on time (Saturday should be ok all else equal) then by the time you are off the plane and picking up the car (some reviews indicate that getting to the car can be a bit slow - Bristol isn't the most modern airport) it could easily be 10.30 -10.45.
Then, say 2hrs 45 to St Agnes - so arrive at 1.30 ish. Really you aren't going to have a lot more time than to see St Agnes and area on the Saturday. Perhaps you could go south via Gwennapp -the Pit amphitheatre is open all the time so, even if the visitor centre is closed you could see that and other aspects on your way to Truro in the evening as it gets dark!
Then you can play it by ear on the Sunday depending on the weather and how early you want to get away as to which places you see on the way back. What sort of places do you use for Hotels? If you want the breakfast (at Mannings you would be paying for it - and how c £100 plus!! That sort of hotel gets expensive for a single.) that would delay departure on a Sunday! Do you ever consider e.g Travelodge prebooking deals?

I hadn't seen that price, a bit more than we paid. However we arranged it last minute on a Bank Holiday weekend in May, prices everywhere were high.

If you can get a deal at one of the chain travel hotels Travelodge then it may well fulfil the needs better.

Devon and Cornwall can be deceptive when it comes to time on the road. Past Exeter the motorways stop, but from our experience last year, I would think St Agnes and around is viable on the first day. And then see what time you have on the second. If you are in a rush to head get back just be wary that traffic out of the West country can get pretty heavy on Sunday afternoons. In March you should be fine, but it it is worth keeping in mind.

The most interesting sites were all open air, so you don't have to worry about closing times, just the amount of sunlight. Gwennapp was just in the middle of a village, I didn't even realise there was a visitor centre. There were boards up around it explaining though. This is where I discovered what World Heritage is in Cornish (Ertach an bys if you were wondering).