I had to make a claim at an Employment Tribunal before I got what I was owed.
My story is probably a lot like yours: both I and a male colleague frequently worked more than 35 hours a week, i.e. more than "normal" contracted hours. I did that work despite being on a part-time contract which pro rated my pay, pension contributions, holiday and bonus entitlement (based on an assumption that I work 30 hours a week). I didn't receive the same pay and benefit for the same 5 hours of "normal" working hours work that my male colleague did just because I was a women on a part-time contract.
Organisations and businesses are unjustly benefitting from free labour provided by women who are engaged on part-time contracts but still end up working full-time hours. And it happens A LOT. Why? A variety of reasons: poor management, a complete absence of management giving a s#£#, a workload too large for the size of a team, an inexcusable acceptance of "this is how it is".
The law should protect women so that if we in fact work full-time compressed hours under a part-time contract our pay, pension contributions, holiday and any bonus entitlement are not pro-rated.
Here are the arguments that helped me fight back:
Template arguments - Part-time Worker Regs, Equal Pay and Indirect sex discrimination
Would love to hear how you get on if you use them too.