TCM0124040 | Eligibility - remunerative work (determining): Charity work or voluntary work (Info)
From HM Revenue & Customs · Tax Credits Manual
A person working for a charity or a voluntary organisation, or who is a volunteer, shouldn’t be treated as engaged in qualifying remunerative work if the only payment received, or due to be paid, is payment for expenses incurred.
There should be no question of notional earnings from this employment.