Compromise Agreement & SA = confused

[agaskew]

Hello all,
Please excuse my lack of knowledge in the area of taxation as I have been PAYE forever.

In 2010 my job was changed, and I accepted the offer to sign a Compromise Agreement, that included various payments described later. This employment ceased on 31 December 2010.

I received a P45 in mid-January 2011 that listed only my regular salary earnings and tax for the period April 2010-December 2010. This P45 does not include the payments made under the Compromise Agreement.

As per the Compromise Agreement, I received the following payments at the end of January 2011. These are the verbatim descriptions from the Agreement:

£45k by way of compensation for loss of employment by reason of mutual termination
£18k by way of a transition incentive payment
£5k Pay in respect of holiday accrued but untaken as at the Termination Date
£100 Consideration for confidentiality

Here is the confusion:

The accompanying payslip for the above payments (dated 18/02/11) states the tax code used for these payments was BRM1, which I understand means Basic Rate Month 1.

Total Payments: £68k
Tax Paid: £7700
NI Contracted in: £300

Now filling the the Self Assessment (presumably because I tripped over the £100k threshold) and entering the above as other payments where the first £30k is tax free, my calculation shows I have an unpaid liability of just over £10k.

My questions:
Why was I taxed at BRM1 and not using my regular tax code?
Why was this stuff not included on the P45?
Is a "transition incentive payment" taxable?

Any help greatly appreciated!