If the tax due in one tax year is over �500, you have to make payments on account to cover the following year's tax.
The first payment on account is due in January, and the second in July. Each payment on account is calculated as half of the previous years' total due.
Let's assume that your husband's first tax return will be due next year, and he owes �1000 in tax. This has to be paid by 31 January. He will also have to pay �500 by the same date, which will be his first payment on account.
He will then have to pay �500 by 31 July, for the second payment on account.
When he completes his second tax return the year after, he will have already paid �1000 towards it. If the bill is more than this, he will have to make up the difference by 31 January (and pay the first payment on account for the year after). If it is less, he can either claim a repayment, or leave the money there to cover future tax bills.
So, you may want to put aside a little more for the first year to cover that first payment on account, as it can be an unexpected double whammy if you're not careful.