For the first time ever, I am having to rehome one of the puppies I have sold through no fault of owner or dog, just unlucky circumstances. Anyhow, I am wondering about the breeding restrictions originally placed on the dog, will they now be void if the puppy is sold onto another home and paperwork is signed over?

No they will still stand. As your the one who put them on you will need the new owner to sign a contract which states what the endorsements are and what if any conditions They would be lifted.

If the old owner (who wasn't the one who put them on) sold the dog on and didn't tell the new owner they would also still stand. If someone sold a dog they put endorsements on and didn't tell the new owner about them (and get proof) the KC can lift it against their wishes.

So providing you tell them and get it in writing they know all is fine.
Here is the full info
https://www.thekennelclub.org.uk/media/8269/endorsements.pdf