How long should I be on a BR tax code for?

I have just received notice from the Inland Revenue that I owe nearly �8,000 for years 08-09 and 09-10. Apparently this is because I have been on a BR tax code for all that time. I started my current job in Jan 07, shortly after which I gave my employer a P45. I had been a higher rate tax payer in the job before this - so can't understand why this has happened. I checked with payroll and they don't have a copy of the P45 I gave them, and I can't find my copy now. They did check on the system though and they do think that they did have a P45 from me though because by the second month they paid me they have figures for previous employment. I don't have two jobs, and I just can't understand how I can have been left on this code for so long (and because of this I now seem to owe a large sum of money). It would seem to me that either my employer or the Revenue should have put me on a proper coding. How should I proceed from here?