I have an ancient pair of Karrimor boots, and they're still sound. Had a lot of wear, lot of abuse, and are now relegated to the gardening, but still very wearable. Himself bought a pair about five years ago and they sort of keeled over to one side in few weeks. Something not quite up to par in the stiffening/linings.
Boots are such a personal thing, everybody's feet are different, everyone's stride is different, and the use they're given can greatly reduce their usefulness too.
Nowadays I wouldn't buy boots I hadn't seen and tried on, not unless I (or Himself) had had that kind previously.
Haven't tried Mountain Warehouse boots, but Himself is happy with their walking trainers. He's hard on those, his daily walk is a ramble and a scramble through woods and fields, up and down river banks, etc., and it kills boots and trainers. Merrell lasted six weeks, MW four months with no blisters, hot spots or twisting.
I'm wearing one of their cagoules just now, and it's been excellent. Better than my Berghaus, Arcteryx and Jack Wolfskin ones for wear, that's for sure.
Their trousers last better than the Craghoppers or other brand names, apart from Rohan, too. I would add that Himself is one of those lean, trim, wirily built men, 5'8" weighing in at under 10stone, so it's not size and weight that's physically destroying clothes and footwear, just wear and tear and hard walking. He's using Brasher and Berghaus boots just now, and seems happy with them, though the Brasher's soles are starting to really wear down.
If the Karrimor boots are a real bargain, and you can afford to replace them quickly if necessary, then you could try them and might well be surprised. If they're not going for a song, and you really need them with no way to replace quickly, then I'd be reluctant to take them. Not with the issues we know that many of them have had.