what i mean by this, is come tax time, when you have to pay it, the tax man expects you to have it ready, he also expects 50% of this years tax as a deposit for next year at the same time, so say you got to pay 1000 tax bill for this year, you ahve to pay 1500 as half is for next year, this was explained to me by the inland revenue.
If April 2009 tax bill is £1000, you don't pay that £1000 till Jan 2010...9 months later.
They also want 50% of that £1000 which is £500 at the same time in Jan 2010, this would be 4 months later as it's earnings from Apr to Sept 2009.
Come July 2010 you are due to pay the remaining 50%..£500 tax, earnings from Oct 2009 to Apr 2010... 3 months later.
If April 2010 tax bill is now up to £1200, you already have £1000 "on account" so you just pay £200 in Jan 2011 + 50% of the £1200 and 50% of the £1200 again in July 2011....
So you never pay tax before you earn it, contary to some folk's explanations