The guy that taught me grafting (I've been two years and still only 50% success so will go again in February!) sells 2-3yo trees that he's grafted and I've had a couple of wee apples off some of the 4yo ones but not yet a full size juicy one. That is on dwarf rootstock, I'd have to check the numbers of the size to be sure. Anyway, he is Dutch, the secretary of the Newburgh Orchard Group and a huge apple expert, so why don't you come up to visit and see what he can tell you. The fruit markets are on Saturday mornings at the moment, selling apples and very likely some of his young trees. Or I can see if he's around midweek to show you his huge greenhouse full of potted grafts!If I remember to dig out his email you can ask all your appley questions of him, esp the central Scotland suited ones, but don't ask him about stone fruit, not his thing!
I have planted Discovery, Bloody Ploughman (local Tayside heritage type), Cox and I am dubious about my James Grieve just now.
Plums you can't get better off the tree than a Victoria - I lost 3 and a bitter cherry to silverleaf and have just one old Victoria growing but other than this year (one fruit survived the frosted blossoms) I usually eat several every time I go by to see ponies or sheep, or just to go pick a plum!
Don't give up on your tatty apple trees just yet, by all means plant a couple more but a good prune can do them the world of good and they'll be there while your new ones grow up. If nothing else, even sour tasting ones left long enough will stew and make a decent crumble..