Why won't HMRC collect underpaid tax through my tax code?

I submitted my 2011 Tax Return on 31/12/11 showing underpaid tax of just over £1000. I ticked the box to adjust my tax code to collect this amount. I expected to get a suitable coding notice for 2012-13 but instead have received a demand for payment by 31/1/12. Is this correct or can I request they collect through my tax code in 2012-13? (PS: As a higher rate taxpayer with car benefits my tax code is likely to be something like 150T normally, before this underpayment). Thanks all.