Update! Although i never received a reply to my email, or any clarification of the requirements, i did receive a letter today from the DVLA saying they had "previously issued an incorrect V5C", and it contained a new V5C with the Motor Caravan designation... woo! Bizarre that it was rejected the first time around. If i were more cynical I'd be tempted to think they reject everything the first time around to keep the number of reclassifications down. I can imagine not many people bother appealing.