I love Tresanton - we have stayed there 4 times now and it is always reliably perfect. St Mawes itself is a wonderful place, with plenty of very good walks (get the tiny Place Ferry from the quay for one of the best).
The decor is effortlessly stylish, every room is different but think New England chic - tongue and groove, muted Farrow and Ball colours, beautiful bathrooms, fossilized ammonites...
The restaurant is also brilliant - probably in the top 3 in Cornwall - fresh fish and local produce with great service and a very good wine list. If you are lucky and it is sunny (more often than not in St Mawes) then you can sit out on the terrace and gaze across the sea to the lighthouse on the next headland with a cup of tea or something stronger. Delicious breakfasts too.
They have loads of pairs of Wellies for trekking through the ubiquitous Cornish mud (Navy Hunters stacked in a stylish shelving unit of course) and a lovely communal sitting room with newspapers and a roaring fire in the colder months. There is also a really nice bar on the lower level.
They also cater for events very well - we had my father's 60th birthday party there a couple of years ago and it was definitely a weekend to remember. We love the place so much we looked at it for our wedding, but you have to hire the whole hotel for the weekend (or you did then) which we thought was prohibitively expensive - but I can see why they do that - it is quite an intimate space so other hotel guests would no doubt feel the place was completely taken over by a wedding.
It is the kind of place that gives me a warm and fuzzy feeling when I think about it - and also makes me want to up sticks and leave London for the South West...