In fact she had sent them the document in question a month before and it had got lost within HMRC's office. She explained:
The (perfectly nice) lady I spoke to said that apparently my original letter had contained an all-important blue slip. The internal mail system only recognises blue slips, rather than, say, people's names. My failure to include the blue slip had resulted in my carefully packaged 35-page document entering a postal abyss, never to be seen again. While I recognise that not understanding the importance of the blue slip was negligent, I had called HMRC and asked exactly where I should send the package and to whom. The package had my tax and NI numbers on it.The reason first given to the lady was that her letter had failed to reach the person in question because it's 'a really big building'.
This got me wondering how – in the absence of the blue slip - I could have got the document to the right person. Clearly sending something registered post is no use. I suppose I could try delivering it in person, but I've just googled East Kilbride and it's a really long way away. Plus, if HMRC's internal posties can't find someone in their own building, I'm not sure I'd have more luck. Maybe the person in question is very difficult to find – her desk is in an air-vent or something.