|>>|| No. 2180
Sorry to ask but have you tried putting the boiler's make and model into Google to see if there's a user manual for it out there? To my eyes it looks as if you could with letting some water into the system to bring the pressure up, but you could also do with checking if her radiators need to be bled (are they making a clicking sound when they turn on?) or if they have their own thermostatic dials on them whether those need to be adjusted.
Also, make sure she has a hot water bottle, a blanket over her lap and a nice cup of tea while you're sorting this out. If she can potter about to keep herself warm gentle movement will keep her circulation going. The human body is not excellent at keeping itself warm when you get to your nan's age.