Thank you so much for your suggestions.
I have restored the course* (see below) and tried a different Admin account and also a different PC, location and browser! The problem is consistent and always takes 15+seconds to load.
BUT I think I may have found a temporary solution
Here is the solution that I have just found that I think has cured the problem for now. If I turn the course cctivity completion settings off i.e. Course Admin > Edit settings > Student progress > Completion tracking > Disabled... then my course loads in the expected time (approx 1 second).
My hypothesis is that for my course with 350+ users in it the Course completion and/or the Activity completion reports or some other associated logs are being generated, most Moodle users might not notice with 12, 20, 30 students in a typical course(?) but for my course with lots of students this seems to be a problem. BUT it only seems to affect Admin and not the Teacher role.
*One other piece of evidence that may be worth considering is that with the course restored, without the users, the course is still slow if Completion tracking is enabled, which is strange.
So although I have "cured" my problem it is not resolved as it now means I can't use the completion tracking which is a great new feature in Moodle 2.
Any comments? Anyone else noticed this problem?