Hi Graham,
While I was researching something for my own needs, I found a workaround that might work for you. Use this trick to redirect the purchase success page (if you’re using Woocommerce) to a custom success page:
https://www.youtube.com/watch?v=1qAXiVu0Bpo
Then use Facebook Conversion Pixel Plugin to attach the pixel to your custom success page! It sounds pretty simple and should probably work for your needs.
Amir