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Hi.1 The 5% surcharge applies to tax for a particular tax year which is still unpaid at midnight on 28 February following the end of the tax year for which it is due. So, if you don't pay the tax due on 31 July 2013 and any other tax due for 2013/14 by 28 February 2014, you will be hit with a 5% surcharge.2 £5,700 is a not insignificant sum. HMRC could start calling you or visting you at any time. If you cannot pay it all at once, pay some of it on a regular basis, ie every two weeks or monthly and you may be left alone. However, there is no guarantee and HMRC use private debt collection firms these days who only get paid by results.
I hope this helps but let me know if you have any further questions.