Baxi Combi 80 Eco keeps cutting out, red button won't push in!?

I have a Baxi Combi 80 Eco boiler. When I turn on the hot water, it will heat up for about 10 seconds then the flame light goes off and stops heating.

I had to increase the pressure recently, as it had dropped to a half while I was away. Now it is constant between 1 and 2.

I read on these forums about the thermostat reset switch, so took off the front panel. There is a red button on the left hand side, attached to a copper pipe, which is sticking out. Is this the thermostat? I tried to push it, but it won't go back in.

Am I pushing the wrong button? Or does this mean there is a different problem?

Any help appreciated, thanks.

JF