bill young

I am quite a newbie to this but am totally confused as to what I need to pay . I completed my self assesment and as expected due to interest rates on my savings being higher i owe money but....... when I completed the return it sai I owed approx £3000 .but what i dont understand as I am not an accountant is when I went to pay it it comes up with asking first payment on account £1500 1st Jan 2nd payment on account £1500 dur 1st July and then balancing payment £3000 ist Jan .and then says I need to pay £4500 by 1st Jan so I am totally confused as I though I only owed £3000 (all figures are approx ) so think I need help .Will some of it be deducted from my private pension ?