Twotwo - There are three different types of tags:
Slaughter tags - this is a single tag with only your flock number on them.
Standard breeding tags - these come in a pair (same info on each). The yellow one contains the EID chip, and the other one is just visual (i.e. no chip). They have both your flock number and an individual animal number (0001, 0002 etc).
Replacement tags (nb, not the same as replica) - these are like the breeding tags, but are only for a bought in sheep (not bred on your holding) that has lost its original tags. They are coloured red.
Note - If a home-bred sheep loses its tags, you may either replace these with replica tags (i.e. copies with an identical number to the lost one(s) OR replace them with a new pair of breeding tags with your own flock number and the next sequential animal number on them. If you do that, you need to record the change in your flockbook.
So, what it sounds as though you've been doing is tagging slaughter lambs with just the yellow EID tag, when you should have been putting both the EID and the visual tags in. The abbatoir have obviously let this go because their EID readers have picked up the yellow tag, but it isn't the right thing to do.
Most small flock owners only use the double breeding tags and don't bother with slaughter tags, even on lambs destined for slaughter. However, if you do this, you should put both tags in.
Does that make sense?