When you register SORN when you finish the SORN and go back on the road, do you have to do another MOT test?
Yes/No, the MOT is unaffected by any SORN declaration or renewal of (free) tax. The MOT expires when it expires, so if still in date it is still valid.
If your L/R is MOT exempt, then you still don't need an MOT.
It's always a good idea to put it through an MOT every couple/few years just to get a second opinion or maybe pick up on something you might have missed, even if it is not a legal requirement.
Thanks :cheers-man