Hi Andy, many thanks for your reply. You asked me if my bike is a 1988 MK1 or 1989 Mk2.
I'm pretty sure it is a 1989 Mk2 (it was first registered in 1989). If it helps, it is in the black/red and silver paint scheme. Does this enable you to shed any more light on what might be causing the problem? I would appreicate anything else you can add!

Many thanks, Paul