Hi there Coomber
The Good News : I have the exact bottle cage in green that you have on your Trekker which you can have for postage.
The Bad News : Just checked out Royal Mail's website, unless I can get this in a box 16 x 16 x 16cm it'll have to go as a medium parcel which is £5.20 second class. It might be possible if it'll go in the box diagonally but it's a tad under 16cm long on its own.
The bottle cage is in pretty good nick, I can get a photo up tomorrow if you like.