"There are countless times very intelligent students have been marked down simply because they haven't answered the question." Can't agree more with that, not that I'm saying that you are one of those Sunnyjim, but I had a good number of students every term having the problem. What they wrote was good, no doubt, but it was not what they had been asked for.
The following is irrelevant to the thread but I thought of mentioning just to do some justice to the good resources of the OU...
The department I did one of my MAs at had a 5* in research, a 24 in teaching quality, entry requirements very similar to those of Cambridge but... when I couldn't organise in a coherent way all the varied information we were given during the first term, the OU books solved the problem, and soon started to circulate between the students.
The texts are quite good and I confess to be very jealous of a friend who was doing a MA at OU at the same time, as I spent 3/4 of my study time chasing the basic books while she got everything she needed via post, and, as the basics were covered, she could spend far more time than I did researching other sources