If a client has come to you and said they don't think they claimed what they should have against their tax in the previous year and they want to know what you can do to help them.
Can we just recalculate the tax liability and are you able to resubmit a tax for a previous year? and then the tax refund will follow if they have overpaid?
If within a year of the submission date you can amend online. eg you could amend a 2012/13 return up to 31 January 2015 but not any before that.
If an older return you can write to HMRC to amend a return provided you do so within 4 years of the end of the tax year eg you can go back and amend returns back to 2009/10 by 5 April this year.