Thanks Brendan - that's cleared up two of my question marks too (5d and 9a, though the "skill" is a bit loose for the first word in 5?)
I'm not sure of the first word in 13d. I've put in a word that I am familiar with, as a specs wearer, but I can't parse the first 5 letters of the answer I get.
Looking at it again, is the "period of over a month" a very roundabout way of saying "40 days"?