Sorted - Had to:
1.Go to a section where I could measure the diameter of the closed zip.
2.online the zip slider sizes are numbered size 4,5,6 etc , those numbers refer to the width of the closed zip track (mine touring panniers, closed zip track diameter = 8mm (coil/spiral)
3. Order the replacement zip slider type and size. A lovely fella at timpsons has offered to put get it back on the track for me.
. ..