You could try this link, Jon
https://www.gov.uk/requests-for-personal-data-and-service-re...Scroll down and make sure you really read everything. It is over 25 years since your mother passed on, so if you have a copy of her death certificate you should be able to get the whole record (as long as the MoD do not decide some of the information is compromising to current military practice). If you are now the next of kin, it might be free, but otherwise it will cost £30 non-refundable.
I am not sure whether this system works for international applicants or just British citizens, so I apologise if this is not going to work for you. I hope you haven't given up on researching with the wait there has been between your post and this reply!