OK, I think I have just won the numpty of the year award. It certainly was a very loud "DOH!!!" on my part when I realised....
So I tried the water trick on the first piece of tape I had cut to size. Nothing, it wasn't sticking. So I tried the other side of it, jsut in case. Again, nothing.
I figured since it had been held in place since yesterday with insulation tape, and I'd already messed around with it, plus it was the first piece of the roll, so maybe it was just a dud piece.
So on to the second piece I had cut to size. Peeled the tape off the backing and hey! this one had adhesive on it! great! sprayed water on the frame, sprayed water on the tape. position tape on frame, hold, squeeze with credit card and a folded paper towel. And it just slid all over the place. No sticking at all. I was getting pretty frustrated by now, and cursing the stinginess that made me buy the cheapest roll I could find (I think it was 99p posted for 3m x 5cm.... but hey, since it wasn't working but only cost me that much, I wasn't too bothered)
I took the tape away from the frame, went to wipe it down with the paper towel and found a bit of something stuck to the tube. It looked like tape, but it couldn't be, because that was in my other hand....? so peeled it, and a lovely piece of almost invisible tape, the exact same size as the one that wasn't sticking, slowly unstuck from the frame....
Turns out, there's a white backing paper on the sticky side of the tape, and a thin clear plastic on the other. The first piece I tried, the plastic had come away from the tape, not the tape away from the backing... the second piece I tried, the backing came away as it should, but I didn't realise that the tape had stuck to the frame and he plastic had come away perfectly, exactly as it should
I've now rescued the first piece of tape from it's backing paper, and it's stuck on the frame as intended. What with all my messing about, I'm sure it's not the best heli tape job in the world ever, but it will serve and at least when it needs changing, I know how to do it next time!