So I did even more digging and I'm confident I've got my answer... I'll share what I've found in the hopes it helps someone else... I'm gonna be using images from the following threads:
https://www.audizine.com/forum/showthread.php/88590-Pics-of-front-door-speakers-inside-(non-bose)
https://www.audizine.com/forum/showthread.php/532910-Want-to-replace-door-speaker
So as far as I can tell, this adapter is the one that other people have used successfully to mount 130mm/5.25" speakers to their door card:
I found one video where someone had to modify there adapters but I think this depends on the speaker. You can buy the adapter separately or you can buy speakers that have them included, I decided to purchase a set of Pioneer's that had them included. Both can be found on eBay.
There are plenty of guides on removing the door card so I'm not going to cover this. Once the door card is removed and you've peeled back the layer of foam to reveal the speaker, this is what you should see:
You should see three screws, remove them and then remove the speaker, you should be left with this:
Now you'll want to fit your adapters. Initially, I wasn't sure how they connected but after seeing images from the guides above, I think I've got it. I believe the bottom two screws go in first followed by the third when the actual speaker goes in:
With the adapter now fitted, you now fit whichever speaker you've purchased along with the final screw for the adapter too:
Hopefully this is relatively clear. If I've made any mistakes, please let me know and I'll correct them. I'll update this thread once I've received the kit I've purchased and list any issues I had.