Sorry, Matt.. I phrased that wrong..
Should have read as although we've had one (New Year's Day) that won't alter things as it's already been granted as 1 day taken (out of the Bank Holiday allotted 8). So it would only be if any more were to fall within the notice period that they would need to be factored in.
If your notice period ends before Easter then I'd say you're just entitled to how ever many 'standard' days holiday pay base don 1st Jan to end of notice date which I suppose is about 1.6 days per month (20 / 12)