Further to my previous, here is the source material for disputing HMRC claims (amazing what stuff people keep in their archives - just have to remember where you put it):
http://webarchive.nationalarchives.gov.uk/+/http://www.hmrc.gov.uk/esc/esc.htm
HMRC delays in using information
If you think that HM Revenue & Customs (HMRC) should have already collected the tax due in your Tax Calculation (P800) because the information had already been provided to it and HMRC have failed or delayed to use this information, then in some limited circumstances HMRC may agree not to collect it.
An 'Extra Statutory Concession' (ESC A19) allows HMRC to do this and it only applies to individual taxpayers who owe Income Tax and Capital Gains Tax. It does not apply in any other circumstances where amounts owing to HMRC are in dispute.
The circumstances are that HMRC should have used the information provided within 12 months after the end of the tax year in which it is received to notify the taxpayer of any arrears.
Time limits
The time limit which applies for ESC A19 is where HMRC have failed to use information received about a source of income, within 12 months after the end of the tax year in which the information is received.
For example: on 1 August 2007 HMRC received information about commencement of a new pension in tax year 2007-08 and the end of that tax year is 5 April 2008. HMRC should have either issued a tax code or sent a tax calculation for the above receipt of information before 5 April 2009, which is 12 months after the last day of the tax year 2007-08.
The same time limits apply for any over-repayment of tax that is made in error and HMRC asks for the tax to be paid back.
If no information has been received then ESC A19 cannot be considered.
Reasonable belief that your tax affairs were in order
HMRC will consider the facts and circumstances of any claim that this concession should apply and decide whether it was reasonable for you to believe (prior to receiving the Tax Calculation) that your tax affairs were in order.
In my case HMRC came back with wholly contradictory (to these criteria) replies. As I said, perseverance and a little outside help...
Mister B