S Viemeister brought next idea :
Harry Bloomfield wrote:

Also check which side the thread goes through the eye of the needle - your
manual should tell you the correct entry for your machine.

I assumed it went through from front to back, but I will check that point.
Thanks...

That could well be the problem. Many of the old black straight-stitch
machines have the needle set to thread from the side, not the front.
All of the modern, multi-stitch machines I've used, thread from the front.

Spot on!

The needle when I first tried to use was inserted with the eye lined up
front to back. There was a couple of spares so I had tried replacing it
with a spare, but inserted the same way. Once turned sideways and
threaded right to left, it works fine - just needs a replacement drive
belt.

Thanks for all of the suggestions, including the one which solved it.

--
Regards,
Harry (M1BYT) (L)
http://www.ukradioamateur.co.uk