It happens from time to time, its reversion, but it is quite unusual for large and small leaves to be present on the same branch or stem. I would remove the stem with any large leaves present.

This is only a young standard plant there are not many established branches
on it, if I remove the whole branch the overall shape will be spoiled but i may have to so that i can stop this happening. The distorted leaves are actually
very much larger with the outside areas of the leaf small like one leaf has been combined by the the two varieties, if that makes sense. thankyou for
your answer, by the way what is reversion.

Reversion just means its reverting to basic form - many plants are bred genetically for certain features, but occasionally the DNA breaks down and you end up with odd branches reverting to the original and not the bred forms.