Steve,
If the person who's service records you require has been dead for 25+ years, then the FOI Act allows anyone to obtain a copy (minus certain restricted docs). If the person has been dead for less than 25 years, then only the next of kin is permitted access (or the NoK has to give permission to the researcher). In both cases you need proof of death and/or proof of NoK.
The Service Personnel and Veteran's Agency website will provide further guidence:
http://www.veterans-uk.info/service_...e_records.html
Rgds
Jonny