Sir,
Which of these should I go for?

[Edited by futurefirst on May 02, 2015]

Aston. http://www.best-masters.co.uk/ranking-master-marketing.html#.VNC1mWSsWwM

So this list says that Warwick, Manchester, and Cranfield have the best MSc programs in Marketing... Would you say that that's accurate?

I'm not sure what kind of data that ranking uses to rate the program, but those three are good programs. Imperial also has an MSc in Strategic Marketing that's pretty interesting... And Cass has a similar program as well.

I would put Lancaster above Cass and Imperial for marketing.

Dear Duncan, 

Would you still claim the same? Is Lancaster still that good in Marketing? 

Yes, I think that ranking is still a useful list. The Lancaster AMM is very special because it's aimed at people with prior marketing experience: I think only Cranfield has an advanced course of that type. For B2C marketing, and I think Lancaster's especially strong.