It depends on when the exams start Kensingtonia; ds has the first of his IGCSEs on 08/05, so leaving revision til Easter is leaving it too late. He is doing little and often and being tested, so we know it has gone in. This is his chance to make sure he knows his stuff and to identify areas where he is not so confident and then has time to get his teachers to go over it again if necessary.
I used to run a weekly revision class for Year 11 from the October half term until the exams, including two full days during the Easter holidays, to make sure they had sufficient time and opportunity to catch up anything missed; to go over exam technique; to hear what the examiners are looking for; and to go over topics they hadn't grasped. This was any of the year group, not just my own sets.
I also did O levels 30 years ago this year, and I have to say (as a teacher as well as an examiner), I don't think GCSE content is a patch on O level, and the marking is positive rather than negative; we credit what they have got right, not mark them down for what is wrong. I did chemistry and biology practicals, and did lots of texts for O level Eng Lit; Chaucer, Shakespeare, Bronte and some poetry as well. Perhaps we did different boards.
From experience as a year 11 teacher and tutor, I have seen too many kids leave their revision until Easter, and then panic at the amount of stuff they need to get through, (the assumption of the ostrich position, head in sand, arse in air was evident) hence the revision classes I ran, and my insistence that ds does little and often now.