I seem to remember being told somewhere-or-other a while back that there are no specific weekend Cambridge-London diagrams that are allocated to be 379s, just that some are and some are not and it differs each time. Is that (still) the case?
If I'm not in a rush on my weekend trips to London, I rather like to pick up a 379, not least to use the free wifi - it would be nice to plan ahead to some degree. (Not for the rest of this month though, given the line is shut every weekend in January!) But so far I haven't seen any obvious patterns.