Evening Jay. I used to work in a large car plant at Burnaston Derbyshire. One of my jobs was body repair, we used to deal with all kinds of damage caused on the production lines.
Looking at the pic I'd say its a very difficult area to remove a dent as you look as though you have dented across the wheel arch character line. Yes I'd agree its a double skin area and this means it will be extremely difficult to push out from behind.
Before I would make any attempt to effect a repair, a partial strip under the wheel arch would be the first thing then see what panels are where and what access there is to work in.
Worst case would be to pull out what can be pulled with the dent puller and slide hammer to remove the dent, lead up where the puller was tacked on the bodywork and file and smooth off ready for a local paint repair.
One thing going in your favour is its a flat colour, not metallic paint so blending in shouldn't be an issue.
