So my quick release seatpost mount gave up the ghost, with the metal straps that loop round the post breaking away. All a bit perturbing. I dropped Carradice a line - now they did say that I'd probably overtightened them, which may be true, but the next day I had two free replacement straps, and a hint about putting an old inner tube round the seatpost which the straps then went round to give a bit more purchase.
Used it this morning and all is rock solid - not been as secure as that the whole time I've used it!