Sounds almost exactly like the problem I'm having with my Worcester 240 - and after searching here I've come to the conclusion that its probably my divertor valve that's become leakly.. have a search on the forum and you'll see what I mean about the diverter failing.
Hello people - I think I've got the same situation as many of the previous posters on here, but I'm really stuck.. I'll explain.
My boiler (worcester) has just recently started to struggle in producing hot water - thanks to these great forums I'm now getting ready to replace the diverter...