Hi MiriMaran,
sorry to throw a spanner in the works but the DEFRA lady was incorrect (which is worrying). All pigs going to slaughter need to be identified with your herdmark irrespective of their age via a tatoo, slapmarks or an eartag. The 12 months rule relates to movements between holdings. Pigs under 12 months old can be identified with a temporary paint mark when moving between holdings which must last for the duration of the journey. These rules come under the current PRIMO (Pig, Records, Identification and Movement Order) which came into force in April 2007.
Hope this is helpful