How easy is it to get a D? Well it really does depend partly on the student, partly on the course and partly on the teacher!
On my access to science course it really varied according to subject and assessment method. The easiest units to get D's on were the three study skills ones. Approximately 50% of students got a D on these, so 9 easy credits. The hardest units were without doubt biochemistry and physics. Only about 15% of students got a D in biochem, and in physics there was one subject where no-one in the group managed it, and others where about 10% managed. It's probably no coincidence that these subjects were solely assessed by examination. If one assessment criterion was missed in the exams, then according to access rules not only is a D out of the question, but so's an M.
Chemistry was also exam based, and about 30% reached a D in most modules, although one of the easier ones saw 50% getting a D. Biology was mainly coursework based, and typically 40% got a D, with exam only modules dropping to 30%. The essay based humanities subjects such as psychology saw about 35% getting D's.
Some students on the access course got mostly D's, and others really struggled to get any. So as a rule of thumb, 30-40% of assignments will marked as a D. The colleges are under pressure from the awarding bodies not to hand out too many D's because they are frightened of grade inflation diluting the reputation of access courses. If uni's find access students are not as good as their grades indicate, then top universities will simply not want to gamble on these candidates in the future.