Thanks everyone. After some more searching I found a place (UK) that will resilver them for £18 each, which isn’t bad.
I also found that they are available from Abingdon Supplies , but at $90 + shipping from the US that’s a bit dear, as I want 4.
I also found details of how to do it yourself, but I’m not too keen on playing with silver nitrate in the kitchen…
Advice from another board was that you can buy Chrome spray, but contrary to the finish suggsested by the can lid, it comes out looking like dull aluminium, not chrome so isn’t really good enough. There is also adhesive backed silver tape, but I’m not convinced that this would work for very long.