After having the gas man back out to my Sime Friendly for a water leak from the main compartment it turns out it could be a pin hole leak in the heat exchanger causing the problem. The water was actually sat above the heat exchanger which led to the vent being checked for flooding but nothing can actually get in via the vent so the heat exchanger is thought to be leaking slightly, the fan sucking it up and out untill the pressure drops too low for the boiler to work meaningwhat water is left on the fan dropping back on the top of the heat exchanger. As a rough guess the guy thought it would be around 200 quid for the exchanger but im not sure if he meant parts only or including fitting, does this sound a reasonable price? He did say if i hadnt just had the pump done on top of all the old repairs he would reccomend a new boiler but because its the last piece to be replaced we virtually have a new boiler anyway. Came out first thing this morning too with a bad cold and didnt charge either. It didnt leak while he was here and just said to watch the pressure guage to see if it drops over the next week and if it does give him a call back. There are some really good gas men out there willing to help out on a cold winter day, not everyone is a cowboy rip off merchant.