The rules for taxing dividends changed radically from 6 April 2016 with the removal of the 10% notional tax credit and the introduction of new rates of tax on dividends. For many taxpayers that meant more tax to pay on those dividends on 31 January 2018. The same will also apply on 31 January 2019.
If you are a higher rate taxpayer and received £30,000 of dividends in 2017/18, £25,000 of those dividends would be taxed at 32.5% meaning £8,125 due on 31 January 2019.
If you can let us have all of your tax documents as soon as possible we can let you know how much tax you need to pay next January so that you can set aside sufficient funds.