Looks like a "city bike" to me rather than an actual MTB. I had something quite similar in the late 80s. That style of bike was popular in the BeNeLux, but I don't know if it ever caught on in the UK.
That plate near the top of the seat stays holds a classical bike rack and a (full-length, steel, fixed) rear fender.
The other braze-on, near the middle of the right-hand seat stay, is a mounting point for a dynamo.
Both these points indicate that it originally came equipped for touring rather than off-roading. Mind you, it did use MTB wheels and tyres.
Basically it was a road bike with lights, rack and fenders ... but it pretended to be an MTB.
Slap some wheels on it and measure the BB height. It could be a bit lower than most MTBs. It'll also probably have a more road-oriented geometry.
It's certainly no Cannondale, but that doesn't mean it's junk. It looks like it's been ridden a lot, so there must be something good about it.
Build it with some leftover parts and see where you go from there. I reckon it'll make a good long-distance cruiser or pub bike.
EDIT : As for the age, I'm guessing mid to late 80s.