I prefer MM106 rootstock because it copes with clay and only needs staking when the trees are young. One variety I wanted wasn't available and I had to take rootstock M26 and it suffers from bad leaning (now too big to renew the stake easily and the easiest way to keep it growing sort of upright is to prune more one side than the other). The more dwarfing rootstocks are also shorter lived and this M26 which is now 24 years old will possibly only give fruit another ten years, whereas our standard Bramley was a mature tree as long as local memory goes back (before WWII). Bob Sherman from HDRA recommend MM106 even for those trees that are being severely trained and pruned, like stepovers and cordons (both very space saving and fruitful training ideas), because of the vigour of this rootstock and health. The other good thing about MM106 is that the stem is tall enough to mow underneath the tree.
Our trees are in a 4x4m grid - the one between the fruit cage and fence has some bitter pit issues and some canker, as it does not get as much airflow. Lowest production was the Egremont Russet, apart from this year where the few fruit that had set made superb, large apples. The russets need water. Discovery is a must, I agree, and apart from this year (no pollination as it rained continuously during flowering) it is always laden. They start in early August and keep going throughout September into October. They do not store though. They turn redder on the inside the longer they are on the tree - late fruit in October looks a bit like the new Redlove. The other best choice for us was Mutsu (aka Crispin) although as a triploid this needs two pollinators, its apples store until after Easter, our last apple. Very big fruit with Golden Delicious in its parentage somewhere.
Most of mine were from Deacon's. Yes a big financial hit at the time, but the cost becomes trivial when you take all the yield over the years into account. Mutsu was the last one to start bearing a lot of fruit. We only had a pound or three for four years, but now we get hundreds of pounds from this tree (none this year). To help with pollination, I grafted a branch from another variety.
A more economical way of creating an orchard is to go on apple crafting days. No particular recommendation, but here is one
http://www.edibles.org.uk/fruit%20tree%20graftingGoogle will find lots more. You learn the craft, you get a good choice of varieties at a very favourable price. I went on a course with Sarah Juniper and my grafted trees are doing well.
http://www.sarahjuniper.co.uk/applefactor/I often get suckers from the rootstock and in principle, these could be rooted, then grafted for more trees, but I haven't done this.
