Hi all
For future reference I have now fixed the problem, here's how...
1. Remove the centre console (just held in by 4 clips), then remove the stereo.
2. Locate TVSS IIIB main unit, should be behind stereo behind a metal plate attached to a large metal crossmember by cable ties (do not confuse
with alarm sensitivity unit located under steering wheel behind kick panel).
3. Carefully bend the metal plate back (or drill the rivets holding it in place).
4. With unit exposed, locate reset button (small green recessed button).
5. Have all key fobs readily to hand, put keys in ignition and turn to 'on' position.
6. Use screwdriver to press reset button (for at least 2 seconds) whilst keeping an eye on the dash mounted alarm LED.
7. As soon as dashboard LED lights up you have 5 seconds to press both buttons on the key fobs simultaneously (got to be quick for this one!).
8. There should be an audiable 'click' from behind the dashboard as you press the alarm fob keys to confirm they have been reset.
9. Carefully bend back metal plate, replace stereo and clip centre console back in place.
Job done and £££ saved at the auto electrician!!!