Baxi solo 3 turning on and off?

The boiler only stays on for around one minute, then off then on again. Whilst on all lights except overheat are lit, whilst off no lights are lit at all but the boiler and fan light flicker. Although it slowley heats the house this carries on until the timer cuts it out whilst we're at work. And then all lights remain off. The system has recently been emptied to fit a new rad valve and and all rad's bled. Does the boiler just need more pressure adding? If so, how do i do this? There's no gauge to tell me what pressure it is currently at.