The official wording from the Scottish book is.....
The Welfare of Farmed Animals (Scotland) Amendment Regulations 2003 (S.S.I. 2003 No. 488), Schedule 6, Part II, Paragraphs 22 and 23(2) state that:
22 Male pigs may be castrated provided the means employed do not involve tearing of tissues.
23(2) If castration is carried out after the seventh day of life, it shall only be performed by a veterinary surgeon in accordance with the provisions of the Protection of Animals (Anaesthetics) Act 1954.
So, just the same Oaklands
Don't know where I got the 3 day thing from