You say "it was part of a broken down old fence" Literally speaking, a plant can't be part of a fence. I assume you mean the fence was incomplete & borken so gave the impression that the plant was on the boundary. I think this is unlikely, although not impossible if it was planted there when there was no fence (or a gap in the fence).
If he is cutting it back on his side it seems he is intending to put up his new fence on his side of the plant so he presumably thinks the plant is in your garden. If you make a fuss about the cuttings it could lead him to become obstreperous about the location of the new fence, & that could make more of a problem for you.
The item you quote says the cuttings should either be returned or offered. Could he not argue he has satisfied the first of these - i.e. he has returned them? I agree he has not done so in a neighbourly fashion but if he does believe the plant is yours he could probably argue that he has satisfied any legal requirement.
Your options are:
1. Dispose of the cuttings & move on;
2. Throw them back into his garden & see what happens (given your description of the quantity, this would be quite a lot of work for you;
3. Spend money getting a solicitor's advice, which I suspect would be inconclusive unless there is some way of being absolutely clear about the correct location of the boundary & hence ownership of the plant.
I know which I would choose!