Header tank overflows due to warm water from the store

Hi,

Please can anybody assist. The header tank on my BMII fils and overflows due to warm water coming from the vent pipe. It doesn't poor out of the overflow more of a drip.

Also the boiler has started to trip out. I have recently had it serviced and the engineer couldn't find any fault with it, I was wondering if the thermostat on the BMI could possiblty be faulty and be the cause