Well that was easier than expected.
DVLA sent us a new V5 today with all the correct changes, confirming it is now a motor caravan and has 4 seats! No hassle, no questions, no inspections, and no cost. Just over 3 weeks turnaround. Just a case of writing a letter, including V5 with changes, and sending off loads of photos to show all of their requirements being fulfilled. (err…)
So now, we can drive faster (vans have a lower speed limit, not that you’d ever know or hear of it being enforced); lower tolls (though severn bridge charged us class 1 anyway); and cheaper and easier insurance as it is no longer a van with build in progress.