Ok,
SO thanks to everyone who has responded.
It is fixed!
It seems that although it was focusing, it was not doing so along its complete length of possible travel.
My father had not used the instrument for several years and it had lain without TLC for that time, and I think that the lubrication on the thread attached to the primary mirror had dried up / seized.
I followed some internet instructions and partially dismantled the focusing mecahnism and discovered that although the primary mirror was moving, the whole mechanism was stopping half way along the threaded rod.
A bit of fettling and greasing later, and I now have a working scope that seems to produce some nice images (although the seeing has been very poor recently in the UK).
Hopefully now I can contribute a bit more usefully to the forum, now that I can actually see stuff
