Apple trees from seed grow into other varieties because the bees will have pollinated blossom with pollen from other apple trees, unless you are unusually fortunate, and it takes a long time to find out!
Best go and cut some twigs, buy rootstocks and graft onto them.
Or stick the twigs into soil or compost to make cuttings, for full size (large) trees.
I agree, Laxtons Superb is a lovely apple.
