See if there are instructions for your make of boiler on the internet? We are abroad and have something similar that we have to pressurised a couple of times a week. Roll on moving back to the UK, where this is unnecessary.
What does that silver metal flexi hose just behind the gauge in your picture connect to? The filling 'loop' for your system should have 2 valves you will need to turn to let water in and to top up the system. it's hard to see from the photo, but it looks like there's one at the end of that flexi hose - there should be a matching one at the other end. The orange/red dome thing is called an expansion vessel.
This is the sort of filling loop you'll likely be looking for:
Parkview Thanks I had just got there myself when your reply can thru! Just to top left of red knob in pic is a valve - I opened this and put water into system until pressure 1.5 and rads getting hot now At least I know how to do it now and pressure may have dropped so low as spent ages last night bleeding air from rads Thank you all 👍