Try Chapter 8 of JSP760 entitled Career Breaks (available on the MOD Intranet). It should contain the info you are after.
The aim of this Chapter is to set out the general terms and conditions under which the Armed Forces offer limited opportunities for Service personnel to take a Career Break (CB) where this does not compromise operational effectiveness. It is emphasised that CBs are granted at the absolute discretion of the Service; they are not a right and will only be permitted where manning margins allow and where they do not compromise operational capability. This means that inevitably some applications will be refused. Personnel on a CB will not be paid and will remain liable for recall to duty in the event of operational necessity. CBs may be for any period over 3 months and up to 3 years. Service personnel management authorities, however, may cap their duration at less than 3 years to minimise gapping and maintain operational capability. For the same reason it may be necessary to restrict the number of personnel on CBs at any one time.