At £600 you would have to save quite a lot on filler to make it pay
Presuming you have a MIG welder.
Get yourself a slide hammer.
Then get a decent selection of thick washers and self tappers or machine screws.
Mig the washers or screw heads to the panel, attach the slide hammer then away you go
With screws or studs you can only pull well in one plane.
Using washers and either a hook or steel cable through the washer you can alter the angle of pull, handy sometimes on curved panels.
I've pulled a few tank slapper dents out of motorcycle tanks this way when the other option would have been to cut an opening in the tank bottom and work from inside.