JSP 754 - Tri-Service Rates for Pay and Charges said:
04.0206. ... a. For a period of continuous attendance of at least 8 hours up to 24 hours inclusive, whether wholly in one day or partly in one day and partly in another, one days pay is to be issued. Only one days pay may be paid within a period of 24 hours so that, for example, continuous attendance from 1700 hours on a Friday to 1700 hours on the Saturday entitles the individual to one days pay."
The national minimum wage is £6.08 an hour for those aged over 21 but (no doubt in noble recognition of the service of the armed forces to their country), the National Minimum Wage Act does not extend to the armed forces, including the reserves.
Meanwhile, in other news: Army Reserves set to expand to 30,000! Recruiting targets increased! Yadda, yadda.