A friend staying overnight last Monday said that she had received a call, purporting to be from HMRC telling her that if she didn't pay up she would be arrested. The caller asked her to press button 1. She hung up. Apparently if you do press button one on your phone you are agreeing for your money to be taken from your bank account.
She was aware that the call was fraudulent and so stopped the call.
So, please be careful. HMRC will never telephone you out of the blue.
I've got another 'keen'... Ouch!