As I posted earlier... the main argument in favour of keeping the 24w limit is because 99% of terminations after 20 weeks are for medical reasons, not elective terminations.
If you have your anomaly scan at, say, 21 weeks, and it shows a baby with a fatal condition, who will not survive more than a week beyond birth and whose physical problems will cause severe pain both before and after birth, you may feel that the best thing you can do is allow the baby to go peacefully and with the minimum pain. Another scenario is the discovery that the foetus is causing problems for the mother which will prove fatal for her. What do you do if you are a mother of two children and you are told that the baby you are carrying is killing you? Do you go ahead anyway, leaving your two existing children without a mother, and a premature baby that may or may not survive, or do you take the hard decision to terminate?
Fortunately I've never had to make such difficult decisions, but there are women who are in that position. By making abortion illegal after 20 weeks, you take away any choice of these women to limit the pain and suffering of their babies, or even to save their own lives.
Apparently, most doctors will refuse to abort a healthy foetus after 20 weeks anyway, unless the mother's life is in danger - and the law on abortion does say that there must be a good medical reason. A lot of doctors feel that in the early stages, the mother's emotional wellbeing is a good medical reason, but by 20 weeks, the woman is more likely to suffer psychological after-effects anyway, so the emotional state is not usually a good medical reason.
It used to be that quite a few late abortions were Down's babies, but that is much less the case now - partly because fewer people with Down's babies choose to terminate, and partly because those who would, accept earlier testing (e.g. CVS/amnio) and thus find out before 20 weeks.
Personally, I think the law should be tightened - I used to think that it should be limited to 20 weeks, but after knowing people who've had awful news at their 20w scan, I don't feel that an abortion between 20 and 24 weeks is something that is performed for frivolous reasons, and that the option does need to be there. However, I think that the law should state that abortion cannot take place after 20 weeks (I'd say earlier still!!!) unless the medical reasons are serious and life-threatening. It shouldn't be an 'oops! I left it a bit late!' option - though it seems very few women treat it as such anyway.