I took castor oil with DS at 38+6 and i worked for me, and could get hold of it quite easily. Now i have been trying to find it but it has been discontinued for some reason and no chemists stock it or anything.
Tesco and all the other supermarkets and big pharmacies don't sell it or will stock it at all, it's only the small individual chemists that will sell it (well around here anyway)
It's disgusting nasty horrible tasting stuff, but i took it anyway with DS think i took about 3 spoons mixed with ice-cream and started having contractions at 4pm, went to the hospital at 11pm. Fully dilated at 4am and had DS at 4:44am. Didn't need to go to the toilet or anything. But it's different for other people. Some it may work for and some it may not.
And apparantly it's a myth about it causing baby distress which then causes them to pass meconium as it's not a nutrient and will not pass through the cord to baby!
If you do decide to take it, i would say only take a small amount and be prepared for the nasty horrible taste and texture. x