Worcester 24 si II?

Hi
I have a Worcester 24si II. It's was installed 9 years ago. Had British gas around on monday for a new insuranace check and apparently it''s full of muck etc. They've quoted me over a grand to get it fixed (flow sensor, heat exchanger, pressure guage and perhaps a control board.

I'd like to do as much of this myself as poss.

2 questions- 1. pressure guage is currently under 1 bar- how do I increase the pressure?
and 2, how do I decsale the boiler?
Thanks