You are correct, the Ransome MG requires RTA cover as would any garden tractor/ride on mower. Pedestrian machines such as British Anzani Iron Horse, Trusty, Barford Atom etc even when fitted with a trailer or riding bogey are still covered by the NFU insurance. This is a specific question we have asked them about.
Terms of the insurance are covered in the BAVCC Safety Code which is sent to all members, copy will be included with April issue of The Cultivator.
Note it is possible to obtain RTA cover for shows and rallies only, these policies do not cover use on public roads where machine would require registration etc.