One of the things I'd like to do while we are here is to stay in a unique building (lighthouse, castle, etc.). I'm flipping through the Landmark Trust handbook to get some ideas. So far, it has been too inconvenient, pricey, non-ideal location, unavailable etc. but I think I just need to make it the focus of a future trip (2013 more than likely).

So:

1) any other recommended websites beyond Landmark Trust for this sort of thing?

2) any specific recommendations?

Needs to sleep 4 and we are coming from Derby (so can reach most any place). Though not unlimited, we can leave budget aside for now. I'll weigh the cost up with other factors.

Canal boat?

The Landmark Trust owns the governor's lodgehouse of the old House of Correction in Folkingham, Lincolnshire, date 1837, sleeps 4. It's on the site of the old castle, put up by Gilbert of Gaunt, and just outside the village. There is a very good chocolate shop too.

Many years ago I stayed at Langley Castle in Hexham, the reviews still seem very good.

Have you looked at the National Trust cottages? http://www.nationaltrustcottages.co.uk
Some of their rentals are actually in the Big properties and they also have several unique smaller ones.

Not a personal recommendation, but I did see on TV a lighthouse in Llandudno converted to a B&B, and a quick look on the net turns up that and another (both in Wales), and also these:
http://www.distinctlydifferent.co.uk/
http://www.guardian.co.uk/travel/200...beach.suffolk1

Not exactly a unique building, but bearing your children in mind, how about a farm stay:
http://www.guardian.co.uk/travel/200...-farm-stays-uk

Thanks for the recommendations and suggestions. I'll have a look.

How about this place at Thorpeness on the Suffolk coast:

http://www.houseintheclouds.co.uk/index.php?pid=1


Or this place at Hartfield which is in "Winnie the pooh country"

http://www.theoldbarnatbolebrokemill.co.uk/about-us