Scammers are on the increase lately it seems. I had a phonecall come through from a nondescript mobile phone number that proceeded to tell me, via an automated recording, that HMRC have a fraud warrant out on me and unless I press 1 instantly (presumably to go to "pay the fee"), the warrant would be issued.
HMRC giving people a polite heads up warning that they're about to be arrested, off a mmobile phone number. Definitely legit.