Jaguar Combi Boiler - System Pressure Fluctuations?

First off boiler is working OK, however the system pressure is fluctuating.

When cold/ambient pressure is approx 0.8bar, fires up OK then pressure rises to +3.5bar (safety valve doesn't lift), then settles down to about 1.4bar.

Have bled the system so no air trapped and there's no leaks.

I'm thinking the pressure sensor is U/S or on the way out, any other alternative suggestions gratefully received.

Can anyone point me in the direction of on-line spares for this boiler or a compatible manufacturer.

Dick Sibo
December 2009