Rates of pay are a matter for the employment contract with your employer and any relevant collective agreement, although your average basic pay must be at least the National Minimum Wage.
How overtime working is dealt with should be spelled out in your contract of employment and in any overtime policy. Your contract should normally specify higher pay rates for overtime, depending on how anti-social and demanding the shifts are.
Staff do not have a statutory right to double pay on Sundays for example, but there may be a right to this written into the contracts of employment.
If there is no reference to different levels of overtime payments but you or others have consistently and automatically been paid at higher rates in the past, then the courts might decide that an implied term, based on custom and practice, exists in your contract of employment.
