what is wrong with my combi ?

HI, i have a potterton puma 100 combi, about 12 year old and when i switch the central heating on,after approx 20 minutes, the pressure guage reads high. then left longer , say overnight, the low pressure light comes on and boiler has shut down(pilot off) and i have to top the system up and re-light the pilot. cant see any leaks. can anyone help please ?