Any question answered...

is it a combi? Has the water pressure dropped below 1psi? there should be a tap to fill the central heating system again.

Google the manufacturer/model etc. Manual should tell you how to re-pressureise. If set up right it's a piece of piss, so easy even I can successfully do it.

Don't over-pressurise though.

Cheers guys. The black needle is at 0.8. Red needle is over 1. I'm guessing the 0.8 is the cold coming in. I'll play with it. Here's hoping that fixes it.