Callum,
Thank you for this procedure, but it no longer appears to work in the current version of the Affinity Photo software. I just tried it and I could do all the steps except for step 6. When you right click on the spare channel that you just created, there is no option to "Load to Background Alpha". Instead there are the options to "Rename", "Delete", "Load to Pixel Selection", "Add to Pixel Selection", "Subtract from Pixel Selection", and "Intersect with Pixel Selection", none of which seems like the correct option for this task.
For reference, I'm using Affinity Photo v1.6.11. Not sure when the change occurred, but unfortunately this solution no longer seems to work. Any other suggestions from anyone out there?