Cordon Tree growing advice please

There shouldn't be a problem planting 8 inches from a fence - just make sure that they get enough water due to the fence blocking rain from one direction.
I'd suggest using M9 or M26 rootstock for most cordons.
If your soil is good or you plan to grow a vigorous variety go for M9.
If your soil is only average (and assuming you won't be feeding etc) then you may want M26.
If your soil is low-fertility, or if you plan to grow a slow-growing variety then you might even need MM106 for a cordon.
("The books" don't mention that the natural vigour of the variety grafted can make quite a difference to growth rate).

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I also don't see a problem with growing a one-sided espalier/fan (possibly cutting off one side of a ready-grown espalier), but I'd suggest at least M26 and probably MM106 rootstock, depending on how big you want it to get. It will also need support or it will gradually lean/bend.

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You should also carefully consider the varieties which might do well in your area. The usual shop-bought varieties are not easy to grow at home and can be quite prone to various problems unless sprayed. Besides: why bother to grow shop-bought varieties when you can buy them cheaply in the supermarket?

Thank you so much. That information is so useful.

I wanted the branches of each espalier to extend about 2-3 metres - is that too ambitious?

I'd like something that wouldnt take too long to make an impact on a boring bare fence, have average soil -so an M26 then.

When growing espaliers, if the desired spread of the branches is (say) 2 metres either side of the trunk, then you should choose a rootstock which is able to reach at least two-thirds of that distance as a free-standing tree.
For example: M26 typically reaches about 2metres as a free-standing tree if grown in a reasonable soil.
Therefore the branches would spread about 1metre in each direction from the trunk.
Therefore M26 as an espalier in a reasonable soil might (eventually) spread about 1.5 metres in each direction. MM106 mght make 2 metres in each direction. If grown as a one-sided espalier you may be able to grow them a little further.

I would suggest M26 for your cordons and MM106 for your one-sided espalier. With such a long, one-sided reach, it will need good support to avoid uprooting or breaking off at ground level!
Remember that the variety which you graft onto it can affect how large it gets. You should also be way about growing very vigorous varieties, slow-to-start-fruiting varieties or tip-bearing varieties (Bramley is all of the above and is best grown as a large heavy standard tree!). That's not to say that they can't be grown, but they will be more difficult than most.
Some varieties make excellent espaliers or cordons due to their natural growth habit and tendency to produce nicely-spaced side branches and fruiting spurs. Some have thick strong branches while others have thin weak branches. Generally speaking, "upright" growth habit means stiff branches while "spreading" growth habit means flexible/droopy branches. "Upright-spreading" is probably best for an espalier. Some varieties are a devil to train the way you want them.

That is great advice. Thank you for helping!