I think a letter from Pete Abbott would suffice. You'd only know the year, rather than the actual date, but if it allows you to get it on the road sooner then it's got to be good.

At least you got more than i did, Citroen just told me to get in contact with Citroen in Spain.

Cheers haha.

I'm not sure that a letter from Pete would suffice - click the link & read the 2nd from last article on the page:

http://www.monkeybike.co.uk/

"Important notice regarding dating certificates, as of the 1st August 2009 only genuine Honda Dating Certificates supplied by Honda will be accepted by the DVLA for registration purposes of Honda Monkey Bikes.

Any certificates issued by Monkey Bike UK prior to the 1st of August 2009 that have not been used for the purpose of registration of the certified motorcycle cannot be refunded by ourselves."

There's a big difference, though, between some random company and a club such as 2cvgb - DVLA have a list of "recognised" clubs, and named individuals within those clubs who they trust - and, IIRC, Pete is one of 2cvgb's official greybeards.
http://www.fbhvc.co.uk/dvla/