Right down to the dropouts, mudguard eyes and chainstay protector this is a dead ringer for the Centurion Le Mans used to have, which had Suntour kit and was from about 89, which AFAIK means it's a Japanese-made frame.
Not the lightest - but would make a solid, dependable winter trainer that will ride OK.
Going by head tube length - it's same size too. Which would make it a 20".
It's well worth £100 of anyone's money. I rode mine to death as a winter-trainer when I had a full licence, until I had to scrap it as there was no metal left in the BB shell to put a thread in.