I have just bought a trials modified Tiger Cub with PVL ignition. It looks like PVL has milled a slot in the casing (where the engine number was, as it's not by the front mount) and added their aluminium logo into this slot, therefore I only have the frame number to go by. Has anyone got any experience of registering a bike with an age related plate based on frame number only?

No, but I suggest you contact your local DVLA, make an appointment for an inspection by one of there staff.

They will most likely ask you to transport the bike to them.

absolutely No way to register a vehicle without an engine and frame no. some people faced with similar circumstances that you describe may re stamp the original number as they remember it at or near its original position.

but no number equals no registration and thats non negotiable.

Thanks for the replies, I have also had an email from Mike Estall, who can help. He can provide me with a dating certificate which should get me going in the right direction.