Hot water is warm, not hot!

I believe that my Ideal Isar boiler has a faulty diverter valve which is allowing hot tap water, into the central heating.

Would this be contributing to the fact that my hot tap water never gets hot, it only ever gets warm at best?

I assume that if some of the hot tap water is escaping into the central heating pipe, then this would be the cause of the problem. (the temp setting on the boiler is set to max).

Could I be right?

Yes you are.

You can confirm this by feeling the heat going to the rads when you are running hot water by holding the 22mm pipes under the boiler. Dunno which one is the flow on that particular box though.