TillySolstice

Thanks for that, I believe I've gotten to the bottom of this!
 
I've found this support article from Apple; https://support.apple.com/en-gb/ht204977
It appears that when you upload a RAW file into your Photos library on your Mac, in order to conserve space on your iPad it creates an embedded JPEG that syncs with your iPad. This process is seemingly also clearing the EXIF data, however Affinity Photo still sees the original 'RAW' tag from the file, hence it shows the RAW logo when opening. This is why when you are opening the file that has been synced from your Mac, it goes straight into Photo Persona, because the iPad is serving up the embedded JPEG.
However, when editing a photo on your iPad through the Photos app, it must use the RAW file and applies a non destructive edit. This is why after editing the image in Photos, then importing it into Affinity Photo, it uses the RAW file (with EXIF in tact) and opens in the develop persona.
You need to change the Download Originals option on both your Mac and iPad, the above link shows how to change this on your Mac. On your iPad open settings, click on the top option on the left hand toolbar (Apple ID) then navigate to iCloud>Photos>Download and Keep Originals.

You may need to remove the image from your Photos library on your Mac, then re-import the RAW file for the changes to take effect, but there is a chance that your Photos on your iPad will update automatically and remove the created JPEG... unfortunately the Apple article is not clear, so I'd recommend double checking first!
Hopefully this will solve your issue once and for all
 

Dear Dan
I think that might have cracked it! two photos have loaded properly now all originals are on the iPad - it is very confusing knowing what is RAW and what is a JPEG on the iPad as it doesn't tell you in the photos app!!
Many thanks for all your time and effort to help me out.  It is much appreciated!
Sarah