Try the Yachting and boating world forums ... Google, "ybw.com" if they don't have a list available they would always be able to point you in the right direction... as they are all in the same boat, metaphorically speaking.

Is there anywhere on the web where I can compare the prices that UK marinas charge for berthing yachts?

I've searched the compare.com sites with no result - it would be extremely handy to have a site where the berthing charges i.e. overnight/monthly/per annum could be compared for all (or most) of the marinas around the UK coast.