There were two problems: my low skill level in this sort of thing is one, and the limit of the sheet size is another. The Wills Craftsman Kits are clever in that they maximise the sheet size on parts. In other words the kit is designed so that there are no major joins. The LH ridge is exactly the length of one sheet. Theoretically this works, but human nature being what it is means that the RH gable could well be a mil lower or higher on the angle. Then you're in trouble as the LH ridge is too short or too long. Deep joy. Here I was too high so that there was a gap at the bottom of the gully that you could drive a Reliant Robin through. The fix was to file the point down and sharpen the angle somewhat, but then the gutter line and the ridge drop out of horizontal. All this while trying to juggle all the bits with masking tape and blu-tak.
The hindsight tip is: if you get one of these kits, leave cutting the chimney hole until the roof sizes fit and give yourself a line or two of tiles at the bottom extra to that on the cutting plan. The length is fixed of course, but at least you're not having to deal with gaps at the gutter line as well, and this could be trimmed at the end.