can I buy a new clip that fixes onto the rail (not clip on slat) and how do I fix it.

Hi Betty
Do you mean for a vertical blind headrail, the runners in the track? If so, we don’t unfortunately – we just sell full replacement tracks which can be found here. Please note we sell 3.5” and 5” spaced tracks.
https://www.blindsbypost.co.uk/vertical-blind-headrail-and-track/89mm-white-vertical-blind-head-rail?fast_search=fs
https://www.blindsbypost.co.uk/vertical-blind-headrail-and-track/127mm-white-vertical-blind-head-rail?fast_search=fs
Thanks 
Emma