Never had it happen before, I suspect a shoddy plastic moulding, but difficult or impossible to prove that I didn't break it by trying to force it.
No luck with RMA to eBuyer, and Seagate don't repair HDDs or sell the bits.
So it looks as though my best option may be to try and glue the plastic back onto the metal.
Superglue has been suggested but (not that I've used it much) I think that it is very quick acting and if I don't get it absolutely right at the first attempt then I could end up even worse off.
So is there something suitable for bonding hard plastic to metal (and a very skinny bit of hard plastic) which allows you to slide the two bits together, reposition to get them absolutely aligned, then hold together for a bit whilst it sets?
Assuming I manage to repair it then I will probably use an all-in-one connector for both power and data to give more rigidity.
The alternative is to glue a modular power cable to the drive as a permanent fixture.