2008 Tax Liability and coding out in 2009.

When submitting 2008 tax returns online, and applying for the tax liability to be coded out in 2009/10 I think the deadline was supposed to be 31st December.
However I'm sure that I read somewhere that due to software glitch HMRC extended this deadline to 31st January.
Am I right with this, and if so can someone point me to the official statement please?
I submitted a few tax returns towards end of January applying for the tax liability to be coded out, but in a couple of cases have received recent statements showing the tax as underpaid (plus interest), and the code numbers without any reference to the tax.
Thanks.
David Evans