Thanks for the replies. The GS isnt actually 'stuck'... it just sits in the middle and moves down gradually from the top of the instrument... and then just stays there moving slightly. Oh ... and I did try tapping it.
I ommited to mention in the first mail that the shop did replace the 'capacitor' in the HSI on the first test just incase this was the problem. Clearly not. My suspicion is that it is something to do with the KN53... but I am not a good enough electronics expert to know what sort of signal would give this behaviour in the HSI.