Faulty Worscester 35 CDi loosing pressure?

My Worcester 35cdi has had 4 replacement heat exchanger's in less than 10 years the first 2 were within the first year of installation the problem is it looses 1.5 bar of pressure in 2 days.
I can top the system water back up but it does the same again, the radiators have no air in them and always remain hot from top to bottem of the radiators when the system is on.
The the hot water has always been fine apart from when the pressure was too low.
I have check under the floors to see if there are or were any leaks nothing all is dry. We did have a period of about 6 years when everything seemed to be OK and apart for an annual top up of very little water at the annual service every thing was fine until last September when it all started again finally the maintenance company replaced the Heat Exchanger again but the boiler has started do the same thing again.
Has anyone out there come across the same problem and if so how was it fixed? as the maintenance guys don't seem to have a clue now.
Any assistance would be much appreciated.
My old Ravenheat Combi of 18 years old performed far better than this one and only ever had to replace the hot-water diaphragm.
Thank you in anticipation.

Philip Teale
January 2017