Yup, tax credits!!

[Beerwench]

OK, I've got myself into a right pickle regarding tax credits and my new job!

Basically, I work on an 8 day rota - 4 nights on then 4 nights off, and get paid weekly up until and including the Sunday, and I get paid for what I've actually earned that week. So some weeks I get paid 3 nights, some weeks 4 it all depends on the days I work that week. Shifts are 8.5 hours. I'm seriously struggling to work out my 'average' hours per month, so that I can work out what I'll potentially earn to update my tax credits. I've put it down as 31 hours a week but that doesn't seem right, when from that I work out annual salary.
Can anyone help me work it out please? I'm terrified I get it wrong and end up with a massive overpayment! And I've tried ringing - used all my minutes on hold waiting!
Thank you in advance if anyone can help.

So by my understanding out of every 28 days (4 weeks) you'll work 14 shifts in a 4 nights on, 4 nights pattern.
Each shift lasts 8.5 hours.
14 shifts x 8.5 hours / 4 weeks = 29.75 hours a week.

I think that's right but hopefully someone will confirm.

8.5 x 3 shifts x 26 weeks = 663 hours
8.5 x 4 shifts x 26 weeks = 884 hours
Total per year = 663+884 = 1547 hours
1547/12 months = 128.9 hours per month
1547/52 weeks = 29.75 hours per week

Thank you both ever so much, I'd got myself in right tizzy trying to work it out but the way you've explained it makes sense.
Thank you