Central heating is not filling hot tank when heating is on?

I have a central heating system which has stopped filling the hot water tank when the heating program is on.

I have tested and if the heating program is off, and I turn the water program on, the motorised value moves and water is piped to the tank. If I do the same test but start with the heating program on, no water is directed to the tank. So I think this means that the motorised valve is working (and it is a 60/40 or at least able to split the feed to both heating and tank.)

Any clues as to why this might have suddenly started happening?

Will
February 2012