The price increase applies from 31st March. If you were previously billed beyond this date you will be credited up to your last billed up-to date at the old price and then billed again at the new price, as well as being billed for the following month.
I’m with plusnet and my March bill was actually from 6 March to 5 April, so on my April bill they recalculated the part from 31 March to 5 April as well as billing me from 6 April to 5 May. The CPI they use is 5.4%, giving a total increase of 9.3%.