Leghorn,
As you're in England (and this is applicable for wales as well) :
The weaners can be moved with a temporary paint mark, this mark only has to last the duration of the journey, and can be anything eg "blue spot on shoulder", "Red stripe on back" as long as you can tie back what is written on the movement form with what is on the pigs.
Once on your holding, the pigs need no id as long as they stay with you, so you don't need to do anything !
We recommend either slapmark or metal eartag just before they go off, as they don't need it earlier. Eartags can be lost. Slapmark is only used in practice for slaughter, and should you circumstances change and you need to sell them live, then you'll regret slapmarking them earlier.
Sally - ID after 12 months (or at any age to slaughter) only applies to a move - so as long as they stay with you they do not need any ID even if several years old.