No hot water! Potterton 24

I have a Potterton 24 combi boiler and the hot water has just stopped working (it's luke warm at best). The Central heating is working fine.

I've tried restricting the flow, water still comes out luke warm

I switched the heating off for an hour, both large (hot) pipes underneath go cold and remain so (burner does not fire when I turn the water on only)

Is this a problem with the diverter valve or something more serious?

Help would really be appreciated.

If you do a search on diaphragm, diverter, and look at the pin, you should be able to find it all.
Also partsarena.com/baxi

Many thanks Chris. Unfortunately I'm not a plumber so the graphs don't really help (cool website though).

I've got a plumber coming round tonight, i just wondered if anyone had an idea what we might be dealing with?

Graphs? Eh?
I don't understand. He'll be looking first at the things I indicated - if you read half a dozen threads you'll see why, I think.