Question for the self employed

Serious question for once chaps,

This last tax year (2003-04) was the first for which I’ve had to file a tax return, as I’ve been moonlighting/freelancing on top of my regular salaried job. The tax return was submitted, and yesterday I got my tax bill (sadly the Royal Mail are all too adept at delivering these…).

The amount for 2003-04 was as expected, but also there’s a calculation for 2004-05, which equates to the same amount of tax as for the previous year, split into two installments, the first payable along with the 2003-04 sum by 31 Jan 2005.

I thought I’d only be paying my tax for the tax year 2003-04 in January, so this extra amount comes as quite a shock, especially as I shan’t have my 2004-05 figures until the end of the current tax year in April!

Is this standard practice by the Inland Revenue, or have they ballsed-up my tax bill?

Thanks in advance…