Allocation of SA tax payments to earliest year

I am sure that a similar question has arisen previously (I'm even reasonably certain that Euan provided the answer), but I have been unable to find it via the search facility.

Apologies for raising it again.

£6K was paid on a/c to HMRC in May 2007 in anticipation of the liability arising from the (late) 05/06 TR. In fact, the 05/06 TR was not submitted to HMRC until Feb. 2011, along with the TR's for subsequent years.

HMRC did not initially process the 05/06 TR and allocated the £6K payment against the liabilities for later years. The 05/06 TR has finally been processed and interest is being charged, and is still accruing, on the unpaid liability for that year. The interest would be considerably reduced if the £6K paid in May 2007 was allocated against the 05/06 liability instead of against liabilities which became due some years later. A call to HMRC asking them to re-allocate the £6K payment proved fruitless, the response being that "the payment cannot be re-allocated because it has been utilised against SA liabilities which were already on the system before the 05/06 liability appeared".

I was under the impression that a taxpayer could request (retrospectively in this case) that a payment could be allocated in accordance with his/her wishes, particularly if this created a more equitable result (i.e. a lower overall interest charge), but I have been unable to find any confirmation of this in the HMRC manuals.

Any guidance on this issue would be greatly appreciated.

Thank you.