I think the well does sometimes fill up with water. The sealing rings on the Ford plug leads are not a complete circle, there is a 3 to 4mm gap in each ring. So water could get down into the plug well. My MOT man says that he sees quite a few cars (fords and fiats) with water in the wells. Take a look at this old thread of mine. You can see the tide mark on some of the plugs.
http://www.talkmorgan.com/ubbthreads.php/topics/349397/1 I did fit a cover and did not have any trouble with water in the plug wells.
YMMV
Cheers
John
