95 GT Speedo problem

I've recently acquired a 95 GT convertible, which is great to look at and even better to drive - roll on the summer!

However, I have a problem with the speedo - the best way to describe the fault is that it works when it feels like it. Usually when starting from cold it will not function at all, after a mile or so it will rise and register 10 to 15 mph, when I'm probably doing more like 30 to 40. Interestingly if I 'drive her enthustically', the speedo may climb at a rapid rate - and read faster than I'm going - usually it then settles down and works fine.

Now all of this wouldn't be such a problem were it not for the mass of speed cameras we have over here in the UK - with zero tolerence. The limit is 30 mph around town so not having a reliable speedo is a might worrying.

Any pointers please?