Seat clamp was perfect, Cyfa - cheers!
The rear brake - yeah, my plan (don't know what you think..?) is to shorten the hose and have the Tektro as a front brake - then I have a hydraulic front brake on both bikes, and - better - in the short term, I can just swap the front wheel. Neither of my frames has rear disc IS mounts, and though adaptors might be a work-around, I'm not fazed by a V brake at the back. It may be my motorcycling background, but I put a lot more emphasis in the brake up front.
So - if it comes to it, for Haldon, I'll swap the cable from the Giant to the Marin, and run the Giant without a rear brake for the time being.
What do you think?