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OP - may well be that the crank arm on the motor has moved round. Arm is normally on splines (unsure specifically about Signum) and held tight by a bolt - 12 mm?
Worth a look anyway... 
That's what I'd suggest. My Astra's wipers are the same set up as the Omega's so see no reason why the Signum would be much different.
Turning wipers on while they're frozen to the screen is a likely reason.

yup.....is it driven by a doris?
Or does it have rain sensing wipers? If left on prior to snow fall over night, and started next morning with snow on the screen the wipers take off to clear the rain/snow and can't, possibly causing this issue.
In winter turn off the rain sensing wipers before leaving the car, and or lift the wipers off the screen all together ESP in freezing rain type conditions.
Loosen the nut on the wiper motor, reset the park position, re tighten the nut to motor. May well need to remove the scuttle for access, which means removing the wiper arms and refitting in the same wrong position to get it right. It's a fiddle, depending on the signum set up.