chargeable gain and new tax code

Newly_retired

I have some underpaid tax due from 2006-7, because of a chargeable gain in Feb 2007 which pushed me into higher rate band. This came to light when I had to fill in a tax form after I retired last September. I am now on basic rate. To cover this bill HMRC have just issued me with a new K tax code for 2008-9. I am thinking of paying off the amount as a lump sum so I can have a clean start next tax year and also because the loss of the 10% band will leave me worse off.
Is this a good idea, or does any tax expert see any better ways round it?

Newly_retired

Thanks Jimmo. Yes I take your point that I am being offered an interest free loan - I hadn't looked at it that way before!
You are right about several changes of coding since September and it still isn't finalised as I have a small annuity monthly payment to come on top of works and state pension - details not yet sorted. The underpayments are
(i) a potential sum of £65 approx for 2007-8 and
(ii) a definite £618 for the chargeable gain - after top slicing relief - in
2006-7. Actually I am really annoyed about that as my IFA advised me to surrender an insurance based investment, and if I had waited till the next tax year there would have been no additional tax to pay as it would not have pushed me into the higher band. If the taxman can wait till 2008-9 to collect that through my tax code, is there any way I could get that chargeable gain spread over into 2007-8, so there was no further tax to pay? ( I have already had 20% tax deducted at source ). I do not understand why my IFA made the recommendation and intend to take it up with him. I suppose I did not have to agree, but what is the point of having an IFA if you don't trust them. I should have listened to my own instincts, but I was so busy working at the time.