You can’t rely on HMRC. Well, perhaps that statement has something in common with statements about the religious beliefs of Popes or the defecatory habits of bears; but the First-tier Tribunal has just reconfirmed it anyway.
Mr Heslop had been a dentist for many years. He retired in September 2010. Having settled all his PAYE liabilities he phoned HMRC to check he’d done all that was required of him; and he was assured that he had. He found out in September 2011 that he had been misinformed and that he was still obliged to file a P35 for 2010/11, which he duly did, in October; and was duly charged a penalty of £500 for its late submission.
Mr Heslop thought that in the circumstances he had a reasonable excuse for the late filing: he had reasonably assumed that when HMRC told him that he had done all that was required of him they meant that he had done all that was required of him. Sadly, the Tribunal (in  UKFTT 62 (TC)) didn’t agree: he hadn’t specifically asked about filing the P35 and he ought to have known that he had to file one.
We’re sure that there is a planet on which this is a fair decision. It just isn’t this one.
Moral: don’t assume you can rely on HMRC. Ask someone you can trust.