Ah... thank you Paul. That makes perfect sense now you point it out. Those bits were vaguely bothering me but clearly not enough to make me read the instruction sheet properly! It does say "upright panels can now be fitted along the top edge of the side flares".
As to why Kieran and I both made the same mistake, builder fatigue might have something to do with it. The relevant instruction is close to the end of a long list.
This should be a joy to fix
Cheers, Alan