What does it say in your contract about over time and holiday?
Employers who have lots of workers with variable hours often use a calculation to work out holiday entitlement, and will often look at average hours over a 12 week period to work out holiday entitlement.
Is it usual where you work to have lots of people doing overtime? The odd extra day here or there, or working late a couple of times, you wouldn't expect extra holiday, you'd just be paid the extra or get time off in lieu.
But if you are averaging a full time week over a month, that's slightly different, and you should ask your boss to recalculate your holiday entitlement based on you working 35 hours a week for that month. It won't be much of a difference for just one month, but still worth having.
If you are going to be doing more hours regularly, you should get your contract altered accordingly. Your holiday will then be sorted at the correct rate, and when it comes to things like pension contributions, redundancy payments and other important things, having your contract closely reflect what is happened can be very important and give you extra protection.