2012. There are two, a little one and a slightly bigger one. They travel at least 3 metres when they spring off and its so much fun finding them again... The opening is a little too small for needle pliers. The answer (for me) was to work from the other side and to push them on. Sorted. Thanks for the responses.
I used circle pliers but they can "flirt" a long way. Put on one side and use pliers to prise the other side over and into the slot. IIRC, the smaller one just pushed on like a C clip but maybe I made that one up.