Probably your pump has died - mine just did the same thing a couple of days ago! Same symptoms as you, hot water but no heating.
My old pump was always noisy so was easy to tell that it was no longer functioning.
My local plumber charged £90 inc VAT for whole job (i.e. new pump and labour to fit). New pump was a cheapo - about £40. Grandfoss ones (the best) are around £75ish.
If you don't have a usual plumber, get a Corgi one off the gas register site and ask them to quote over the phone for a new pump first to make sure they won't rip you off, as I've seen people charge £200 - £300 for this job!!!
Andy McCarthy, September 2009
