Hi i know if you have a sucker below a graft it will be the original rootstock

I've never heard of a branch from the stock growing from a branch above the graft - wouldn't have thought it was possible. Have you checked carefully that the branch in question does actually grow from a branch above the graft - from the photo I could imagine that it disappears behind the thicker branch and could have grown up from much lower down, concealed by the trunk.If this is what's happened it might well be from below the graft - can you crawl under the lower branches and check if you can see its origin?

Me neither, I did wonder if it is from below the graft or even a separate plant from a dropped plum that has grown.

the only other thing I can think is that there has been a mutated bud on the main trunk that has then grown without you noticing it. After all that is how you get a variegated stem on an all green bush and vice versa.

Thanks stera and sbg, ive had a close look and traced the branch down to what looks like a previous branch pruning ,(pic1) it seems to be from the centre of a 1.5inch cut .Its definitely not a graft as i planted the original sapling .You can see in the second picture it running parallel to a branch.I think you may have something there sbg a mutated bud sounds possible.
Sorry ive inadvertently deleted the original pics.

Well Jake, now you are peering into the world of microbiology and of course plants are living things. Nobody has all the answers, except God. There is such a thing called a genetic anomaly. That's just a fancy term for the plant doing what it wants to do and we still have a lot to learn. In short - I don't know but sometimes truth is stranger than fiction.

Your right Paul ,well the genetic anomolys fruit tastes good so i have taken three cuttings hopefully one will take,if not ive got the pit to try (that may be different though) :0)

lol