Apologies, I should have been clearer in my post.
Receipt of money from an employer doesn't affect illness benefit payments, because it's not means-tested. For example a person may continue to be paid by their employer while out sick from work.
Regarding holiday pay, it's the dates you're paid for that are important. If you claim illness benefit, and you receive holiday pay for any day covered by the medical cert, that day is not counted as part of the claim and you won't be paid illness benefit for that day.
If the holiday pay was in respect of dates prior to your claim, they should not affect your claim.
From the guidelines on Illness Benefit:
Neither Sunday nor paid holiday leave are treated as a day of incapacity for work while any two periods of incapacity for work separated by a period of paid holiday leave are treated as one period of incapacity for work.