https://www.moneyclaim.gov.uk/csmco2/index.jsp
Complete this and send it to him. If he is in the army he should be pretty easy to track down. The current tennants should have contact details for him (or his managing agent).
Your claim will set out the basics...
1 We (all of you should just claim together) rented place from date to date
2 As part of this rental agreement we paid a deposit of 1275
3 We gave notice on date and terminated this lease on date as set out in the lease
4 The repayment of this deposit was requested on date
5 The landlord (defendant) requested proof that all utility bills were paid up to date before he repaid the deposit monies.
6 All utility bills were paid up to date and the utilities changed to the name of the landlord on date. This information was sent to the landlord on or about date
7 To date only 255 of the deposit has been repaid.
8 We are claiming 1020 of the deposit plus interest due (any set out in the lease) or statutory interest at 8% of (work it out to the date you submit the claim) amount due to you all 1020 X 8% X Days he has had your deposit (from the outset)/365 and 2 pence per day until paid.
If you have done everything as set out this is a pretty open and shut case, he should either cave and repay, or if he wants a fight will have to counterclaim why he is witholding (and has 14-30 days to do this). All you guys will need is proof of the actions above.
To file will cost you GBP30.