Puma Potterton 80 pressure gauge up and down?

Any advice welcomed....
A week ago, my boiler stopped working because of faulty pressure switch so disconnected and boiler then fired up.
Since then, the pressure has been moving between 0.5 and 3.5 when the heating is on.
Just came home this evening and the heating is not working - although the hot water comes on fine.
Pressure has gone into the red so drained.
Is it possible that I have a leak although the pressure does go up and down without any drainage.