Ian,
If you Google "copper P clip" there are loads of suppliers out there e.g.
http://www.remora.net/catalogue/MICC-cable-gland-fire-clips-tooling-gland-shrouds-saddle-clips-p-clips/bare-fire-clips-and-saddles/bare-fire-clips/bare-copper-fire-clips
This page gives:- Clip Size, Cable Diameter Range and Code for the clips.
https://www.tonesties.co.uk/shop/product-details/bare-copper-pyro-cable-p-clip/