@Si Mcinerney
Ok, so the vid covers the main steps involved. However, the car they used doesn't have the roof pump which will be in the way of the aerial base if installed. It can be loosened off and swung out of the way - no need to remove it entirely. I'm a bit hazy on what I did and didn't take many photos, but anyways..
1. Disconnect battery
2. Pop the clips holding carpet in place on left of boot, pull carpet back to access inside wing. Undo and swing roof pump out the way if applicable
3. Undo the earth strap from the aerial base and undo the bracket that holds the base in place. I don't think I disconnected the wires to the base as they did in the video
4. Aerial base should now be loose and only held in place by it's connection to the exterior rubber grommet
5. Find a screw with the same thread as your aerial (think they're quite common, I had one lying around from a pound shop set of mixed screws), screw it into the base and push down on it to free the base from the grommet. There is also a rubber boot attached to the aerial base that you have to free from the wing.
6. Alternatively you could try and brute force it by pulling the base from underneath. At your own risk though - I did number 5 above so have no view on this alternative..
7. Remove grommet by either pushing it out of, or in to, the wing
8. Install new grommet, matching the curve of the wing with the curve at the base of the grommet. Position the bigger bit of the grommet towards the front, which should ensure the aerial correctly points up and back when installed. Also ensure the wing is properly seated on the inside lip of the grommet. The grommet should lie flush all around and makes a satisfying 'pop' when it locates
9. Offer up the aerial base to the newly-installed grommet, insert the magic screw into the base and pull up on it to locate the base into the grommet. Again, a satisfying 'pop' should be heard if it locates correctly
10. The reverse of steps 1 to 3 for the rest of re-installation
Think that covers it. Take photos so you know how everything goes back together. Shout if you have any questions