I've now added the remaining rain strips. Seeing as a picture paints a thousand words and all that, I thought a few photos might help to explain the process.
Firstly, I marked the centre of the roof and then how far the rain strips needed to sit above the gutter. I also marked those heights at the end of the roof, and then taped the strips down in parallel. Then a dab of thin CA to fix the strips in the centre.
I then removed the tape, marked the heights of the strips above the gutter at the ends of the roof, curved the strips into position, and taped them down. The tape was deliberately placed where the water tank covers need to go, so that length of the upper rain strip wouldn't get any glue (it needs to be removed later). A third hand would have been useful here...
And so the strips are in place with a (reasonably) sweeping curve down to the ends of the roof.
I already had markings in place for the water tank covers, so was able to mark and remove the short sections of rain strip either side of the tanks, and then fit the covers. I gently curved those between my fingers first so that they weren't trying to spring away from the roof while the CA grabbed.
Not perfect by a long stretch, but better than with the overscale strip I'd used before I think. And paint should hide most of the howlers, with luck.