Glow-worm Ultimate - topping up tank in loft?

How do I top up the pressure on my Glow-worm ultimate system?The controls are on, pilot light on but radiators not getting hot. I have been advised to top up the pressure.
Thank you a
There are two tanks in loft. One with a stop cock. This looks OK. Is it adviseable to top this up further?I have just knocked the pipes coming out of the boiler and this seems to have fired the boiler up and the radiators are getting hot but obviously this is not the correct scientific way to repair it!