Just thought I'd resurrect this thread to see what solutions people have found for this problem.
I've had to remove a tag from one of our Zwartbles, since it had started to work loose and was causing her pain. I'm having the same problem as the OP now though, in that I need to order a replica tag, and don't know how to do it!
The ZSA yearbook gives two options, but both involve having both government numbers on one side of the tag (e.g. UK123465 001122) and the Zwartble number and year letter on the other side (0123-100B).
After recommendations on here, we decided on Shearwell SET tags for our other sheep, which worked really well.
However, I can't order SET tags with both the government numbers on the same side of the tag, so that leaves no room for the ZSA information except for in little letters on the end of the tag (where the Canadian flag is on this one):
So can I ask what other people do? Is it really the case that a two-piece tag is a prerequisite for the Z's? I can't think of another way round this that wouldn't result in a lot of wasted DEFRA numbers each year due to the year letter changing.
Thanks in advance!