Thanks guys. Yeah, interesting about the BS batteries. The one I picked up (not too spendy for Halfrauds) is factory activated. I'd thought it was somehow second best to the BS variant, so that all sound great. I gave it a top up on the Optimate before installing, and everything seems fine. So all set for the MOT next week.
It does feel a bit jerry rigged, but it works. I will replace the strap though, when I get a moment.
What's bugging me now is resetting the clock. The set button isn't in the full flush of youth, and is more than a bit hesitant to make a clean contact. Is it wort trying to clean up the contacts, or is it time for new switchgear?