They also say this on their website;
"If you are not aware of the required information and therefore cannot complete the required criteria you can telephone the Certificate Services Call Centre on +44 (0)300 123 1837 to place your order or write to Certificate Services at PO Box 2, Southport, Merseyside, PR8 2JD."
Is it not worth writing, (or even better, persuading one of your contacts in Westminster Archives to phone), giving a couple of sentences of explanation about Mary and this thread and the diary, and seeing if they can come up with anything?