Very nice rifle! Looking at "British Gunmakers, Volume One - London" by Nigel Brown I found the following:

1940: SN 18,227
1947: SN 18,262

So, based on this it looks like your rifle was built between 1940 and 1947 but probably closer to 1940. The book says that enquiries about the records can be made through J Roberts & Son, 22 Wyvil Road, London, SW8 2TG. Tel: 020 7622 1131, Fax: 020 7627 4442. Then it goes on to say that the records are now held by John Rigby & Co. (Gunmakers) Inc., at 1317 Spring Street, Paso Robles, CA 93446 and that the records are now held there. Tel: 805-227-4706. If the records are now held in California, I am not sure why the author left the old information there. Maybe there are records in both locations? I doubt it though.

Regards,

Jim