You're not too far away then John, just along the A40.
As JT said above, are your other gauges correct (well, its a Morgan, correct'ish)? Do they fluctuate with engine revs?
There is a screw on the back of the dash (nice thing with these earlier Mogs is that you can get to the back quite easily) to which all the instrument earth leads go. It's worth checking that they are secure and that the screw isn't about to fall out of the (wood) dash.
Mechanical dyslexia? You have an ideal car to learn a little about motor vehicles, even if just the simple things...