I think the biggest problem with trying to boost the 335's power lies in the fact that the original gun was aimed squarely at the German market and even at the time it was introduced, Domestic German airguns were comparatively low powered, mainly because there's no hunting requirement for them over there. Even the ones made for export to "higher powered" markets such as UK were only ever putting out about 9 ft/lbs. Perhaps it's just not designed for it?
I think I may just have buggered up a low powered, but very sweet shooting Haenel 303 by ignoring my own logic It remains to be seen.
The South of England has 2 good things, the M1 and the A1. Both will take you to Yorkshire.