Try an electric impact wrench. Here I'd get one from Argos, but not sure about HK.

They're cheap, but extremely good, as it seems the THWACK! of the thing is far more effective than leaning on a bar.
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MC21SP Plaything
BMW F800GS Bumblebee
Triumph 9551 Daytona Big boys toy
FJ1100 Sporting relic
GTS1000 oddball