Oh no , sounds like your struggling with the same gremlin
I'll relay my progress and hopefully one of us will find the solution.
Have you had a diagnostic ?
It's interesting you mentioned damp weather , it does seem that the problem occurs more so in damp or wet , although it is cold and icy here today and the problem was quite bad.
I notice your car has stellar mileage compared to my mere youngster at only 133k
Really though , there is nothing like a Merc