How long can you leave grapes on the vine?
 

I grow a number of vines in a cool climate (southern England) and very occasionally the season is very late or there is a larger crop than usual.In England we are quite happy to obtain Brix levels in the range 15-18,and then chaptalise by adding the appropiate amount of sugar.This year we have a very large crop and a late year.So the grapes are just not ripening as usual,so I am wondering whether I can just leave them on quite late.The Brix levels in mid October are about 14,and the lower leaves (Double Guyot training)are only just beginning to turn.It is also predicted that the weather over the next couple of weeks will be very showery and a mean daytime temperature of about 15C(59F),with nighttime about 9C(48F).Some bunches are dropping off,but 90% seem healthy.Will the grapes continue to ripen all the time the leaves are green,and the grape bunches stay without much rot.
Michael