The trunk has split at some point, either through damage or from water sitting in the crook between the trunk and the branch to the left of the image. As the tree has grown, the split has enlarged; if you've seen any weeping, or the area is unpleasant smelling, it might have bacterial canker. But the worry is, that's a deep crack and might threaten the stability of the tree, meaning it may break and fall onto someone.

A good tree surgeon should inspect it - it may need to be removed.

What's with all that string and plastic wrap farther down the trunk, though? It looks like it is cutting into the bark, which will cause even more problems.

Good point, Tug, I managed to not notice that at all...

Thanks for all your answers. just by way of information I was advised to put that sticky tape and tie the string tightly to avoid a type of moth that will attack the fruit as it apparently travels up the trunk from the bottom but the tape prevents it getting any further. Should I take it off then?

Might be a bit late to worry about it now that the tree's got a possibly serious problem further up the trunk, it might need to come out anyway. But if you mean Plum Moth, there are other ways to deal with that see here

https://www.rhs.org.uk/advice/profile?PID=554

Being wrapped tightly in such a way won't be doing the tree any good, but you do need to get it checked for safety.

Good Morning Bamboo, Thanks for the link, I will check it but I think it will have to come down, its only a dwarf tree and it has never had more than a few plums in the 6years since it was brought.....and they were all black in the middle, each year we had hoped for better but its not to be.
Thanks once again for your help.