To be fair, them cheap dent pullers probably wont go near a 20cm long dent so I would be inclined to save my £15. They're designed for 10p size 'dings'.

The cost will most likely treble if you have to go through a garage but if you can find a local Mobile Paintless Dent Removal firm (a quick google of 'dent removal <city or town>' will show up loads) and he can gain decent access to the rear of the panel, then it should cost about a hundred quid-ish in my experience. It's quite a skilled job nowadays with the types of paints used and thinner metal used on modern cars, it's easy to make it look worse and even break the paint with a DIY attempt...

Best for "<<DENTDEMON>>" (eg...) to see if he can do it first than head to your local bodyshop or get the toffee hammer out I would say...

JJ