Great idea, thanks lowpugV2.
I know the blower motor is ok, but I can easily isolate the speedo for a test.
It's an old car (32 years old), so lots of things can go wrong.
A few days ago the problem deteriorated to the point where it would squeal when revved in neutral, so I looked in the engine bay with the engine running and confirmed that it was the one of the drive belts - most likely the water pump and alternator belt.
The belts were tightened and now there is no more squealing!
There could have been an as yet undiagnosed cause such as a problem with the alternator causing more drag, but for the moment the problem is solved.