As Lillian says they should already be tattoed for their breed registration.
If they arrived older than one year, then they should have been eartagged if in England and Wales for Defra regs, or a temporary mark if under 1 year. In either case, you don't need to add any more ID as long as they stay on your land.
If you are taking for meat, then when you take you will need to eartag with your herd no. Tattoing the ear with herd no. will not meet abattoirs needs, and slapmarking on a Mangalitza is probably not easy enough to see (have not taken any to slaughter so may be acceptable).