can’t find this anywhere! Anyone know the part number or spec of this clip which has disappeared?

That's not available as dedicated spare... mine failed too at a point and i used one which fitted well from a E-clip box i had in my garage; like this one https://www.ebay.co.uk/itm/120Pcs-3...302280769302?_trksid=p2349526.m4383.l4275.c10 ... or measure the diameter of the channel and buy a proper one, iirc it was the 4mm one?? ..i'm not sure cos i picked it by eye