We would always advise you to make a disclosure to HMRC of any errors in your tax affairs. By doing so you minimise your potential exposure to penalties and maximise the abatement process so therefore the penalty that you're likely to receive from HMRC will be far less than were HMRC to discover itself.
Behaviour is everything
Under the new penalty regime your behaviour is everything. An error caused by reasonable care is considered far less serious than one where the error is deliberate and you've then concealed.
Speak to an expert
Within any behaviour category for penalty purposes there's a minimum and maximum amount that you can receive. Were you to accept that your behaviour was deliberate, that you knew at the time that your tax returns were wrong when you delivered them to HMRC we work with HMRC to ensure that you get the lowest possible penalty within the minimum range.