wondered if anyone had succeeded in agueing that a 12 hour a week post grad course designated by the university as full time is actually part time (p602 of current CPAG book)?

It has been successfully argued for Carers Allowance that what the college said can be overruled by the student’s personal pattern of attendance, depending on the reasons. Have a look at the commentary to reg 5 of the ICA Regs. I would think that the same principles underpinning the case law for CA regarding whether the college’s view is determinative or not could apply to the means tested benefits.

The rules for colleges of further education and for universities and other higher education course are carefully drafted in different ways for IS and JSA. There is no reference to "16 hours of guided learning" for HE which is the test for FEIt is the course which has to be full-time and not the student's attendance on it (they may just be lazy or not doing the work).
The case law indicates that the teaching institution's view is not conclusive but should not be disregarded without very good reason. Cases include R(SB) 40/83 and R(SB) 41/83.
One useful pointer might be if the university also offers a part-time course and the comparison of hours of teaching time.
Some of our older universities always used to regard lectures as optional and only a couple of tutorials a week as compulsory, but I doubt they would ever have regarded their courses as part-time. Undergraduates are expected to spend a lot of time, even during the vacations, reading and studying alone, and this would be regarded as part of what constitutes "guided learning" - ie, homework - on an FE course.
All the legislation and case-law is so different from CA that I doubt CA rules are of the least help.