Posted: 03/03/2016 at 14:20
I would start with a 35 litre pot with your M26 apple trees (I'm guessing they are 1 year old maidens or 2 year old trees?). You can always pot on in a year or two. You can buy black plastic pots off ebay which come with handles which allows you to move them a bit more easily.
They should be fairly stable if filled with John Innes No 3 and if you want a little more stability then you could partially bury the pot into the ground.
I grow apples on M26 and M27 (along with M9 and MM102) and I wouldn't grow M27 apples in a pot to be honest, it's quite a weak grower and I would tend to agree more with what the RHS website suggests (M26)
Keeping it in a pot will restrict the growth and with a decent pruning regime you can keep the tree well within bounds - plenty of summer pruning once your framework is established. Discovery is a partial tip bearer but not to the same extent as say a Worcester Pearmain and will still produce plenty of fruiting spurs. Scrumptious is a good apple.