heating will only come on if the ext water option is selected?

It sounds like you have a valve problem in your airing cupboard, if you have 2 zone valves then the ch one is not switching, if you have a 3 way valve, then it looks like the microswitch on this is not operating, either way it seems as though the valve is opening so you will most likely only need to replace the operator (actuator)

gary.wright53@googlemail.com
March 2008