Old thread, but the scammers are still there with extra tricks.
A neighbour called me yesterday worried by a call from HMRC saying they were about to be prosecuted unless they coughed up £2000 rapidly.
She told me the number that they had been called from (good old 1471) and the "secure" number that they told her to ring them back on, which turns out to be the HMRC helpline number. Bear in mind, yesterday was a bank holiday, so a staffed helpline is unlikely.
Technical stuff now - the 1471 number has a string of hits on watchdog style sites as malicious, usually with HMRC refunds involved and them wanting bank details.
Ringing an answer bureau pretty much by definition cannot reach a specific person directly, so thats a mental alarm bell. How does the malicious caller direct the call? They don't. After they tell you to call them back "securely", they don't clear down, they just listen to you dialling, wait a few seconds, and "answer". The unwary, now secure in the knowlege that they are talking to HMRC, hand over their worldly wealth.
So the answer to that one is to call the number, but to do it on a line that has dial tone, and actually listen for said tone.