Hi.. anyone with any combi-boiler experience??

I have a Keston C36 that has had an intermittent problem with the supply of hot water. Basically it won't switch over to hot water on demand.. though we can somethimes 'force it' by also having the heating on or switching the water demand on and off several times.

We had a new hydroblock fitted at considerable cost - but this didn't seem to fix the problem at all.
Then by a different contractor (who knew more about Kestons) we had various electronic and control panel parts fitted which seems to have fixed the problem.

My question is... did we need to have the hydroblock fitted in the first place? Could a faulty hydroblock have been responsible for the intermittent hot water fault? It didn't seem to fix it once it was out in?

Any help would be appreciated..

Thanks
MFK