I'm thinking of buying a new bike, and would like some general buying
advice. I currently have an ancient moutain bike with 21 gears - basic. My
riding is probably 50 miles a week at most, with a mixture of 50% off road
and 50% on road.

I've been looking at off roaders with suspension. I'm looking to spend a
minimum for something decent - maximum budget is probably £150ish.

What sort of off road? For canal towpaths, forestry tracks, etc
suspension isn't needed. Front suspension is worth having for anything
more serious. As suggested by other posters £150 is not enough for a
new bike. At that price point the components just don't work as well,
need more frequent adjustment and wear out faster.
For a front suspension mountain bike spend at least about £300. For
the extra cash you'll get a far better bike that's nicer to use and
lasts longer.
Try looking in a few local bike shops. If you say what area you
live in someone on this group should be able to point you at a
reasonable shop.
Iain