I’m still going with the polytunnel and have almost got the base rails fitted. It is only almost because yesterday evening, after a smooth patch, I hit problems trying to fix the rails to the door frames.
The first nail went in smoothly and I optimistically expected to wrap up before the sun set. However, the next two nails stuck and started to bend. I decided to drill a larger hole — the instructions said 4mm but, checking back, had earlier suggested 6mm for use with the same gauge of nails at an earlier stage —and got the last one in easily.
This morning I had to hacksaw off the the bent nails and managed to get another one screwed in place. The last one is still hanging out for another round though. Looking at the wood, one of the problems is that the frame is very knotty at that point. In future, I will be looking out for the knots before I cut bits of wood to length to see if there is any way I can avoid getting them under the bits I have to fix together.