The Ministry of Defence has published an employer toolkit providing guidance and support on all aspects of employing reservists. You can read it here.
The toolkit sets out the rights and responsibilities of reservists and employers and the financial assistance available to both parties. It contains guidance for line managers on how to manage reservists and on managing requests for time off for reservist training. It also provides guidance on and template documents to be used when a reservist is mobilised, demobilised and returns to work.