Corrected the angle of the lower side rod leg between the first and second bays because it was pointing inwards too much. This was fairly easily achieved by sanding the joint surface where it meets the main siderod, but also necessitated re-drilling the gusset bracket lower bolt hole. It is now in a much better position so it was worth it as I can now fit the cladding panel.
Next I prepared the new lower panel by cutting a slight groove roughly 1″ from the bottom edge to facilitate folding the panel round the bottom frame rail. The actual angle of the fold is quite severe and I had to take great care not to cut too deep a groove and ruin the panel, especially at the ends. However this was entirely successful and the panel just needs rolling to fit.