Mustard - if your were retained 9as it says on yr payslip) that means you were not FT - retained = lads who work in the butcher/bakery/candlestickmakery who turn out for fires when the siren/bleeper goes.
You had, or could have had another job, also paying you a wage. My best man was an accountant and partner at [big firm] and also a retained firefighter in a village in Oxon.