I have found this site which mentions one cause for exactly this problem:
http://www.bbcl03736.pwp.blueyonder....0%20Faults.htm
he says:
Symptom:- No N. E. S. W. operation from the handset. Powers up OK, and GoTo works.
Cause:- Usually a fault in U12 which will need replacement. These are supplied by various manufacturers, Lucent and Exilinx being the main source. This failure is probably linked to the hot-plugging scenario with the handset.
I have an email outstanding with him to find out what "U12" component is. If I make more progress with this I'll ask a local guy to help with the fixing I think, I'm ok with soldering but not so good with the kind of detail required for PCB's.
And I have an email outstanding with Don @ Bintel.
Not expecting a reply from either for a couple of days. Hopefully one of them will lead to something.
I have a suspicion I'm going to be wanting a new hand controller. We'll see.
Roger.