UK contractors — are you actually putting aside enough for your tax bill?

Something I see constantly with contractors and freelancers in the UK is the payment on account trap.

You do your first Self Assessment, you owe say £3,000. Fine. But HMRC also quietly adds a second charge — 50% of your bill upfront toward NEXT year's tax. So you actually owe £4,500.

Year two rolls around and people are genuinely blindsided.

The rough rule I always suggest — put aside at least 25-30% of every invoice you receive into a separate account. Touch nothing in it until January.

Has anyone been caught out by payment on account? Would be good to know how common this actually is.