Fitty wrote:

We went to France about 3 years ago and brought back some Walnuts. I
stored them but they went all mushy and horrible. I chucked them on the
garden hoping that the birds would like them but one of them grew.

I now have a lovely little sapling but it's growing underneath my monkey
puzzle tree and right behind a retraining wall so needs to be moved.

I'm guessing the best time would be in the autumn or spring but is there
anything else i can do it help it re establish it in another part of the
garden?

I would hate to lose it but if left where it is i know it's going to
bring down the wall, which is about 5ft high.

The height of that wall is unimportant, what's the height of your
sapling, how old is it? If it's less than three years old move it
now. First prepare a new home. Next water your sapling well so it's
easy to dig and so an earth ball will hold together. Dig it up
carefully with a large earth ball and place it on a piece of burlap
and lift the entire thing into a bucket for easy transport. Bring it
to its new home and plant it a little higher than the depth it was
growing. Mix organic matter with the fill soil and fill the hole in
loosely (do not tamp the fill). Water well and tap the sapling to
release any air bubbles. I don't know where you live but I would
fence it to keep the critters from having it for a midnight snack.
Keep it moist but don't drown it, plant roots need to breathe. By
transplanting now it will be well established by winter. Of course
odds are you will be planted before that tree produces nuts.