Low Pressure Light On

Hello all,
I have a Potterton Lynx2 that has been working happily for years. Then noticed that it wasn't working tonight.

The "Low Pressure" light was on, so I turned on the little tap under the boiler to raise the pressure reading to about 1.5. Also bled the upstairs radiators.

Have clicked the "reset" switch repeatedly, but the "Low Pressure" light keeps on coming on. I'm at quite a loss as to what to do next. Could someone help, please, and is there somewhere that has a diagram showing the physiology of a Lynx2, so I know what the different bits are?

Thanks for any help. )

potterton website ?

or the low pressure switch is full of cr*p
easy way to test it but i got my a*se kicked last time
try a search