Tax return 'Payment on account'

I have started my tax return for the first time ever :think: I need to add in some benefits when i get my P11D, but the summary is as follows below. I have removed the actual figures. I get that i need to pay £A, But what is payments £B and £C and why is £C due July 2014?

It looks like they are asking me to pay toward what they think my tax will be next year in advance? If so, does that mean my actual liability this year is £A and in paying £B and £C, my bill on my next return will already be mostly paid?

I thought i had been clever this year by putting the tax back into an isa to earn a bit of money for christmas. Next year i will be paying in advance and losing out on that. Can i opt out of payment on account and keep the interest for myself?


Or have i completely misunderstood?? Any advice appreciated

Apologies the formatting below goes a bit odd, hop you can make it out....
Jamie

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Summary


This section provides you with the result of your calculation. Please check the figures and amounts due carefully.
Please note: Unless you are viewing a previously submitted return you must click the 'Next' button at the bottom of this page to save then submit your return.
Total tax, Class 4 NIC and Student loan due for 2012-13 £A

Plus

First payment on account for 2013-14

£B

Payment due by 31 January 2014

£A+B

This amount does not take into account any 2012-13 payments on account you may have already made Second payment on account for 2013-14 will be due by 31 July 2014

£C