When and what you get paid should be in your Contract of Employment ? If you start a shift on a Friday night and that is say a Bank Holiday, then ALL of the hours that are worked throughout that shift should be paid at the appropriate rate, say double time. The employer should not be paying different rates until you have finished your shift. Then a normal rate would apply. Seems like your employers are taking liberties here, unless of course it is stipulated in your Contract. Get a copy and then you will know what is what !!