It sounds like the wheel arch liner it in the way, so you'll have to remove that before you can get behind the dent.
A Dent puller may be able to pull that out in minutes.
Thats an easy job for paintless dent repairer. Would probably look at £45-£75 to pop it back out and most local services come to your door. Rather than trying yourself and potentially making it worse, I would think as a value for the car and cost to repair, its better to get it done by them.
There is no DIY fix for that. It needs a body shop or just leave it as is.
If your body repair skill level is such that you have to ask, anything you try and do will make it worse and will look sh1t.
Can you get anything underneath the arch to push it out? Block of wood & hammer maybe? Try tapping it gently.
Removing the panel will help you out to, if that's possible.
before you start banging at the panel from behind and making more imprints in the paint , get yourself a electric paint-stripper heat gun , place one square of tin foil over the dent and heat it up ,wear sum garden gloves and massage the panel from behind ...the panel should or may show sum movement..you can find this solution on you-tube , how to remove a dent..
Anything you do to it will make matters worse,sorry
How to fix this dent in my car, caused by another car.
I am looking for a quick and cheap method, I know the price in body shops will be quite expensive.
So i'm for a fix on this dent,
http://postimg.org/image/w1svkuecd/
http://postimg.org/image/a02lastul/
http://postimg.org/image/a02lastul/
Thanks