How can I fix my Vokera 24 Boiler?

My boiler was showing error 07. I changed the primary thermistor switch as described in the installation and servicing instructions. I then refilled the system to 1.5 bar (the boiler at this point switched on and was working fine), then after half an hour the pressure went down to 1 bar. I then refilled the system back up to 1.5 and left overnight. In the morning I checked again and it had gone down to zero.