Saunier Duval combi - central heating OK, yet no hot water?

Very useful to have these forums, so can anyone please help, or is it the changeover valve, as someone else's problem was..?
The heating works fine, but when hot water is demanded the boiler fires as normal yet doesn't produce any!
It's in a rental property and although the tenants are very patient and understanding, I want to get it fixed ASAP.
Could it ONLY be that valve, or something else? Is it easy? Expensive? Can I do it? Where would I get the required part?
Many thanks for any help...
Cheers,
Nick