Thanks for the more detailed answer.
Ok, so as your total income will be £34,000 salary plus £4600 state pension, plus main pension £6150 and the two smaller pensions total £548 - we have a total actual income of £45,298 which is in excess of £42475 which just tips you into the 40% tax bracket (by £2823)
You are awarded £9440 allowances plus the flat rate expenses of £60 - which equates to an initial award £9500 (Code 950L)
From this they have deducted an equivalent of a whole years of state pension - this is due to the fact they want to be sure the right tax is deducted between now and 05/04/2013, and absurd as it seems, by including a whole years worth of state pension (when you only have just under half a years actual state pension to be paid) it ensures that the right tax is deducted for the remainder of the tax year.
So a deduction for state pension will continue forever, as its the only way they can then allow the payment to be paid each week in full (without any direct deduction of tax) so by covering it by personal allowances, this deals with the fact that tax CANNOT be deducted from it, as the DWP do not run payrolls on state benefits.
However you also have the added complicated, that your combined incomes see you sit just into the 40% tax rate band, whilst no one income in its own right breaches £42475.
So each of your income has to be looked at, so some of this 40% tax is collected. And its right that the two smaller pensions are charged 40% (so of the £2823 - £578 will be correctly charged at 40%) but this still leaves £2245 income that needs to suffer 40% tax.
If HMRC were to charge 40% on the £6150 pension, which would ensure that the 40% tax due on just £2245 is collected, then you will actually pay too much tax, as only £2245 of the £6150 needs to suffer at 40%.
So instead they make an adjustment in your tax code, to recoup the balance.
I know it seems you are paying excess tax, but HMRC can either choose to really make you suffer by charging the larger pension of £6150 at 40% - which would be too much tax, or make this adjustment in your tax code, to recoup that additional 40% tax due. And they have chosen the option that sees you with as much money as possible in your pocket and not theirs (although you then can claim a refund if they deduct more than is needed)
But if you would prefer to pay too much ( and see it as savings) then you can ask
1) That code D0 be operated on the £6150 - which will see this monthly payment decrease by 20% more (as 40% tax will be paid) and lose the adjustment for the rate bands in your tax code (so your code increase to 106T) OR
2) Keep the adjustment in your tax code and pay the additional amount through your salary.
But in the main I can advise these adjustments to your tax code are accurate.
I hope this helps you understand the need by HMRC, but feel free to ask any follow up questions.