low water pressure F28 lockout, Vaillant Turbomax plus 828E ?

Hi guys, Hope you can help. I've viewed all the relevant and some of the irrelevant posts on similar subjects but nothing fits my exact problem.

I have a five year old Vaillant Turbomax plus 828E. It has worked flawlessly for all of that time until the last 3 weeks. When the central heating turns off on the time clock the water pressure drops to zero and when there is demand for central heating or hot water the boiler locks out. F28 fault. Top up the boiler from the filling valve to 1.5bar and restart and everything remains fine until the next time it shuts down again.

If I run the central heating on 24hrs it continues to work although when the thermostat has signalled off the water pressure plummets to about 0.8bar. I've checked the whole circuit for leaks, nothing. I've checked the PRV out let, not wet. When the boiler is going full chat and the flow temperature is above 50degs the water pressure climbs.

I have no idea where to go now. Could the expansion tank be faulty? or is it something simple?