Leaking Wallstar Combi Boiler?

My boiler has been mended with Araldite a couple of times over the past few years but has recently started leaking again. Am going to try and patch it up once more with Araldite but how do I get the pressure back up? Is it the tap attached to the flexible hose or is it the tap/valve marked '3 Bar' underneath the pressure gauge? Please help me. I'm a bird and I know nothing about such things.