1: if temp control is not working, they are linked direct to pcb, boiler should generally still modulate on whatever temp setting you select but 88 seems a bit high=) if hw sensor is at fault you tend to get hot cold hot cold running water as the sensor does not tell the gas valve to modulate the burner down and results in the burner firing on full flame in bursts like 20 secs on 5 secs off etc etc.
2: If when you run heating the guage rises pretty quickly up and beyond 3 bar resulting in water being released, you have a flat (needs re-charged) or burst expansion vessel. They big red disc behind boiler, valve access from top, same as car valve, push pin in - little air=needs re-charge if you get water=needs replaced or a remote one fitting elsewhere.
ste
October 2009