Losing pressure in a Chaffoteaux Britony Combi SE 80?

My Britony Combi SE 80 keeps losing pressure, when I boost it with the filling loop its ok for a short while but soon drops again.

I have noticed that when there is demand for hot water and/or heating that the overflow pipe is leaking water.

When the HW is on it constantly cycles through hot-cold-hot-cold-hot-cold etc etc.

Last year it had a new pcb and diverter valve plus a couple of other bits but has never really worked properly since (we lost faith with the plumber at that time after nearly 3 weeks of changing parts) but I'm fed up with having to use the electric shower to fill the bath!

Please can anyone help?