AFAIK, the process is almost impossible to replicate without vast quantities of spent yeast.
The yeast strain does make a difference though. Marmite has, over the last few years, released several limited editions made with different yeasts. The Guiness one had a definite beefy flavour to it, the champagne one was horrible, the Marstons rather bland. I have yet to try their latest XO offering.
The yeast difference may also be one of the reasons why I greatly prefer Marmite to Vegemite, but that might just be because I'm British, not Australian, and the advertising and exposure early in life ensured I was a committed Marmite eater (love the stuff - eat it every morning on toast).