You're spot on, Titchy.
Part-timers lose out because leave is calculated the same, regardless of whether you work 24 hours over 3 days or 5. You still get 24 hours x 4 weeks, but the BHs are pro rata. This gives 38.4 hours of entitlement for a 24 hour week.
For someone working 5 days pw, this 7.68 days, but for someone working 3 days, it's only 4.8.
To add to the confusion, there are several methods of pro rating BH hours for p/t workers, and they all produce slightly different results.
I will be in the same boat as you in a couple of weeks, as I'm reducing my hours to 24 pw over 4 days, one of which will be a Monday. The way they work it at my place is by adjusting the annual leave, but not including the BHs in it, iykwim. It's a bit immaterial, as I get 31 days leave, plus another week unpaid which takes it up to 36.
DP is a payroll manager and shit hot at doing this sort of calculation. If you want to post your days and hours, I'll ask him to check it for you when he gets home!
It's a bugger to get your head round, I haven't yet found a way to explain it to colleagues that's easy to understand. And when we had an innumerate manager, she used to ask me to do the calculations for her!