Thanks for the replies. I have given the Probike and the Raleigh to the two teenage boys I have in my foster care. They have an appetite for destruction and both those bikes are very solid! The Townsend has a gear change problem so will look to fix before deciding what to do with it.
It has all got me thinking though that I would really like a good quality retro MTB for myself. Budget of around £150 any suggestions please? I would prefer no suspension, lightweight and good for a middle age man of average fitness!
The brands mentioned above all made some good quality bikes, as did many others, but, if I were you, above all else, I would try to get a bike that's a good fit for you. Unless you already know what sort of bike frame measurements suit your own proportions, that will probably entail looking locally for a bike you can see, sit on and try before buying.
As a general rule of thumb, there is a trade off between comfortable geometry and low weight: the lightest bikes were race-oriented, which entailed a long, low riding position and that is not especially comfortable for many of us. On the other hand, the comfiest, most newbie-friendly bikes tend to be the relatively heavy entry level ones. Additionally, mountain bikes changed rapidly over the 1990s and later 1990s' bikes tended to have more racy geometry than earlier 1990s' bikes—even at entry level. Again, though, that's only a rough rule of thumb. Most of the extra weight in the entry level bikes is in the components, so it is possible to get an entry level bike, swap all of the components for lighter, better ones, and make some headway toward the best of both worlds, But it's a lot of faff.
Bikes like a Marin Palisades Trail, Saracen Tufftrax or Specialized Rockhopper should meet your budget and offer a good compromise. If you're looking at light off-road riding, I would also consider some of the 1990s' hybrids: Marin Stinson, Specialized Crossroads, Trek Multitrack, Dawes Synthesis . . . These can be great value, versatile, and the 700c wheel size looks like it will be around for longer than 26". Last, tyres: cheap, heavy tyres can ruin the ride on otherwise very nice bikes; lighter, better tyres are transformative.