Buy an electric knocker gun off ebay for 25quid, the wolf one, it reallys gives some when it gets going and works well!
Shame I'd done mine by the time mine come, lol, we used a massive pair of pliers and held the top mount retainer plate then used a spark plug socket and a big bar, see my project thread..
And it came undun easy.. You will have to pull rubber off to get to plate properly..
I tested the knocker gun on a mega tight wheel nut, I stood on bar to tighten it a bit more and on the 2nd go it span it right out

well worth the money...