Pressure loss

I have a potterton suprima boiler and a systemate 2000.We were woken up overnight 2 days ago by quite extreme pipe hammer and a rushing sound of water through the system,this lasted for about 2 mins then stopped.System pressure on the gauge had dropped to near zero,although no water had been expelled through the PRV outside.I checked the EV which had a charge of just under 1 bar and no water was coming through the valve.I refilled the system to 1.5 bar and it held pressure for 2 days with all the rads working fine,last night the same thing happened .Do you have any ideas,many Thanks in advance.

On refilling the unit / system
Did you check for air?
At all the points required.
Mike
MGC/H2O