Ok, infact, I've just had a read of their website again and it seems to be showing a lot of information that wasn't given clearly to me at all before.
https://www.gov.uk/vehicle-registrat...tered-vehicles
So from that what I can make out is that if I send off the V5 for my current frame with the new engine number in it then they will require an inspection on the bike.
Am I right in thinking that there is no way for them to match the suspension, axles, transmission or steering assembly to my existing registration mark? Because as far as I can tell if I tell them that those parts are original to the from that I am using then they have no way of proving that they aren't, and as such the bike would have enough "points" to qualify to keep the registration mark for that frame.