Just to warm up the topic a bit first - would you say that, if you accidentally sent a part to someone on here, would they be entitled to hold on to it?
Theft Act 1968;
A person is guilty of an offence if they dishonestly appropriate property belonging to another with the intention of permanently depriving said other of it It doesn't matter how the goods are appropriated or the initial appropriation is dishonest or not, at the point the person became aware that the goods were intended for another person and they refuse to relinquish them it is theft.
You must offer to collect or have it collected with no cost or inconvenience to them.
If it was of little value I would ask if it was of use to them, otherwise it remains mine and I would expect to retain ownership. You dealt with them before so surely they are reasonable?