Hi Flemming,
Okay,don't rub it in about our weather! :-)
If there were no obvious signs of corrosion at those plugs it is unlikely there but worth opening the pins up a little. The only other problem it may be is engine speed. The idle speed should be around 700-900 rpm. You can't measure this without the test equipment but it may be worth tweeking a little either way to experiment. If the engine speed doesn't match the load demands of the drive system it won't go and this is a fairly common problem.
Regards from a less windy but still rainy Britain!