Ravenheat CSI 85 Combi Boiler - Pressure Keeps Dropping?

I have a Ravenheat CSI 85 Combi Boiler. The pressure gauge keeps dropping down to between 0 and 0.25 bar.

I have read these posts that talk of the Pressure Relief Valve:
http://www.diynot.com/forums/viewtopic.php?t=83966
http://www.diynot.com/forums/viewtopic.php?t=37170&start=0&postdays=0&postorder=asc&highlight=

I’ve left a tin under the Pressure Relief Pipe to see if water is escaping here, but nothing.

I’ve taken the front off the boiler and I can’t see any leakage inside.

If I look at the bottom rear left of the boiler (with the front off), the clear condensing box is quite full of water. Not sure if this is where all the water is going? I can’t really tell as the pipe connecting this is plumbed into the kitchen sink waste.

I flushed the CHS two weeks ago and added inhibitor. Bleed the radiators several times. Could the dropping pressure somehow be related to this?

Any ideas greatly appreciated.