Annualised vs variable salary potential discrimination?

[Snow85]

So I work in the public sector. The place I work is open longer in the summer and shorter in the winter. My full time equivalent colleagues are paid an annualised salary so their wage is the same every month regardless of the change of hours between summer/winter. I have just moved from a part time, part year contract where I only worked in the summer, to a part time, full year contract. My hours also change between summer and winter. However, I've been put onto a variable hours contract so my wages change - higher in the summer, lower in the winter. I start at the same time each day as my full time colleagues and finish at the same time. The only difference is that I only work 3 days to their 5. My hours do not fluctuate week to week. I am contracted to work a set number of hours every week and that does not change (apart from summer/winter). I even have set days every week. I recently asked my HR department why part time employees are paid differently to full time employees and they said that we are not - granted, at the end of the year my overall earnings will equate to the same as if I had been given an annualised salary, but being on a variable hours salary makes it much harder to budget, it makes applying for loans and mortgages harder and I am now wondering if it has any impact on my civil service pension contributions? I've spent weeks trying to comb through as many articles and documents I can find online in regards to part time employment rights and annualised contracts but I'm going round in circles. Can anyone shed any light?
Is there a chance this falls under protection of part time worker's law? If the majority of part time worker's tend to be women, would this be possible discrimination? Literally same job, same responsibilities and duties, same daily hours, different method of payment.

[ULA]

Being a part-time worker, you are protected from being treated worse than full-time workers doing the same or similar jobs with your employer just because you are working part time. The key areas in which part-time workers should get the same treatment as full-time employees are:

1. Pay rates
2. Holiday entitlement
3. Pensions
4. Other benefits
5. Training and career opportunities.

There are some others but those are the main ones. I am not sure that differing payment methods come under that. I know many establishments where some workers are paid an annualise salary and others are paid on an hourly rate and some paid monthly. some weekly. If you are being treated the same in all respects of the factors detailed above as your full-time colleagues, then that is the important factor.