With the help of a few pairs of hands, we managed to get it to close, enabling me to drive home. Not had it up since, fearing burnt out motor or siezed runners or the like - until today.
It seems the last time it was lowered, the main body of the drivers side section hadn't folded in as it should & became trapped in the runners thus tearing a fair few chunks out of the material.

Further investigation reveals the roof mattress had shunted forward a couple of inches, blocking the vent hole in the little round flap by the hatch, thus making the fabric puff out rather than collapse inwards when dropping the roof.
Lesson learnt. Keep that little hole clear. It does a good job!
Luckily, my silver roof tent is about the same colour as gaffer tape so a discrete repair will be fairly straightforward. Just be aware that this can happen & can be costly.