historic SA underpayment

The time limit for making assessments and for record keeping reduced from 6 years to 4 years.  However, has this affected HMRC's ability to request payment of an old assessed amount?  I have a widow client.  Her husband died 2 years ago.  She has just received a demand for payment on a 2004/05 assessment, a little under £2,000.  I have acted for these clients since 2007 and have never before seen any request for this payment so it seems to have popped up from nowhere.  Does anyone else have any advice on whether I can make this disappear or are HMRC within their rights to persecute this widow?