I will try and make it easier for you Neil.......lol

If you turn the boiler off, then remove the front cover.....usually a few screws.........

You will see a red vessel at the back i think it is......this is the expension vessel.

On it will be like a tyre valve that you use to blow the tyre up with. Stick a pump or gauge on it and see what it is reading. It should be 1.5bar........

If it isnt, pump it up to that