Whilst your diligence is to be applauded, taxes aren't cut and dry. As you say there's a £1k allowance and there's also a roll-over aspect. The fact you've "made" a few hundred pounds as a one-off here really isn't what HMRC are worried about. It's the person who has been renting out a second (and third, fourth & fifth) home in Cornwall for £4k a week for the last six months that they'll be angling for.