WBLDon - here's the contact for AG&L - http://www.agl-uk.com/contact#.VxBK_fkrKUk

Email them and they'll give you some info on your gun and info on how much a letter will cost.

Based on the s/n ranges in Don Master's book Atkin, Grant & Lang your gun was probably built between 1910 and 1915 but only AG&L can tell you for sure.