My advice is to double-check. The chimps at HMRC are notoriously ****e which is a complete joke considering they have the ability to incorrectly take and ruin lives. Tax should be relatively simple. The first £7475 is Tax free, the next £35k (ie. £42475 total) is taxed at 20% and the rest is at 40% (unless you're CDS!).
Get your total PAYE payable gross salary (less about £30 uniform tax relief per month, up to £243 per month if you get MoD Childcare Vouchers, LOA, Service Charity (Dincome), Service Days Pay Giving and Home to Duty) and pop into the calculator here
UK PAYE Income Tax Calculator 2011 salary calculator UK. Updated for 2011 / 2012 tax year. Calculate wages pension national insurance and student loan repayments online.
I found that I was overpaying PAYE by a massive £400 per month. Three 40 minute phone calls to HMRC later and I'm now paying the right amount of Tax. It seems they don't understand the vagueries of our allowance system - I guess we're too small these days for them to learn about our allowances. I even had to explain my AFPS75 lump sum that was supposed to be Tax free - thankfully, they checked and that is safe!
Spend an evening getting your pay statements out and do the maths; it will most likely pay off. If not try offsetting with the last 5-6 years of motor mileage allowance - about 15p per mile, which if you haven't claimed adds up normally to a lot.
Bonne chance!
LJ