can anyone tell me why my tank in the loft is overfilling?

I have a Baxi back boiler,which works fine,but when it fires up the tank in the loft starts dripping water from the overflow pipe.This only happens for a short period of time,then stops but it starts again when you turn the thermostat up.I have 2 tanks in the loft,the larger tank feeds into a smaller 1.the smaller tank then drips out of the overflow pipe.I have renewed the ballcock and thought I'd solved the problem,but alas not can anyone HELP!!.Many Thanks