Andy_20
Your first port of call should now be JSP 760, which covers all forms of absence, including unpaid leave, from the introduction of JPA, for all 3 Services. If you have access to the Defence Intranet, use the "search" function for "JSP". The top option will be the JSP catalogue (in blocks of 100). Select "700 - ", scroll down to 760 and select. On the next page, select "Launch Document".
If you don't have access, ask your PSF for help (although they might be a bit busy with data migration for JPA at the moment!).
Do be aware that taking unpaid leave may mean that the time won't count towards your reckonable service for pension/EDP and some other entitlements. Read carefully.
Regards
Ginseng