I have hot water overflowing into the loft expansion tank?

I had a new cylinder fitted in December 2009 and the plumber also upgraded the 3way valve to an electric one. A cyclinder thermostat was also added and a room thermostat installed in my dining room. What I now get is when the domestic hot water is being heated, straight from the outset, a stream of water is ejected into the overflow tank. This doesn't occur when the heating is on on its own. What I then have to do is bail out the excess water every 5 days otherwise it just overflows out of the loft overflow pipe. I recalled the plumber and he has turned down the Grundfos pump to setting 2 but this doesn't seem to have made any difference. I have been waiting for 2 months for him to return but I'm now getting anxious that he may not! Can anyone give me advice on what to do next. Thanks