Try writing to the following who will send you a Declaratin of Kinship form, complete this and send a cheque for £20 (search fee). They'll get back to you but it may take up to 3 months.
Ministry of Defence
CS(RM)2
2 Bourne Avenue
Hayes
Middlesex
UB3 1RF
They may need full name, regiment, and service number.
(from PRO site)