Withholding tax

If you have a small amount of tax to pay in Jan.'15 for the 2013/14 tax year then you will get away with the delayed payment for the coming year.  But if you have a significant tax to pay at that time -- and I don't know where the threshold for 'significant' is -- then you will be asked to start paying your tax for the 2014/15 tax year at the same time.  If you have income covered by PAYE then you can have your tax code adjusted and have more tax retained on that income.  If you don't, then rather than simply paying what you still owe for 2013/14 you will be asked to pay 150% of that, with the extra 50% being treated as an advance payment for 2014/15.  And you'll have to pay a similar advance payment by the end of July.  The idea is that if your income is steady then when it comes to settle up for 2014/15 in Jan.'16 you'll owe nothing more for that year.  At which point it makes little difference whether you've paid the tax in advance that way, or via PAYE, or via tax retention at source.  And while you may not owe anything for 2014/15 in Jan.'16 because of those advance payments, you'll still be asked to start making advance payments towards your 2015/16 tax bill.

Nobody likes to pay any bill -- tax or otherwise -- before it is due.  But we shouldn't delude ourselves by thinking the deadline for filing a SA tax return is the time for paying the tax due.  If the total tax to be paid is small, then HMRC will accept the payment of the balance due at the time the return is due.  But if the total tax due is large, then HMRC will want most of the tax to have been paid well before the tax return is due.  Anyone with a significant amount of income paid without retention of tax will find HMRC want the bulk of the tax due on that income paid well before the tax return is due.