I also thought a Roman Villa, but there's no need for them to be near water. Though I do know that Chedworth is build to incorporate a natural spring.

I've just looked through all 57 pages of attractions on POE for inspiration but nothing has leapt out at me  

Ah well, keep thinking. 

Hopefully this will be your last clue .....  Think ecclesiatical building ?????????????

Not UNESCO that I am aware of, but Abbey and Minster are very close to the building you are looking for and also create sites with the STATUS you are looking for..... You are getting very close now Ruth

I hope they don't cos you have put so much work into this you really deserve to win it

Cathedral is the correct but which one and why the disparity between its status and size???

I've only got my phone with me so this is a guess not researched. I driven by this place hundreds of times and it's only just come to me. St. Asaphs Cathedral in Wales, it has Cathedral status but is very small. I believe the smallest in the UK. I think the town was officially given City status quite recently. 

That's the best I can do whilst I'm sneaking a few minutes here whilst we are visiting friends  

You are so so close now, not St Asaphs

You will just have to tell your freinds this is more important

OK. I'm back, one of the few Cathedrals I've never visited is Ely, known as The Ship of the Fens. It's a very large building for a very small city. I've seen it from a distance and it's quite impressive.

I'll start looking elsewhere for when this is wrong