In regards to the original poster, yes, it appears to be very difficult to actually find access to the Transit e-visa mentioned in MSC's e-mail dated 4th November. We have seen several mentions of the $50 transit visa online but actually submitting an application seems near impossible. I was in London on Thursday and actually went to the embassy - the guy there said don't worry, you can buy one on arrival for 200. Assuming he meant 200 SAR then that would make sense as this equates to $50.
Anyone actually managed to obtain a visa in advance?