For marmite fans, it is sometimes referred to as umami, the Japanese fifth taste. You can get in jars in Waitrose.
When I was 19, I had strips of steak we cooked ourselves in a kelp broth in a Japanese restaurant in Vancouver.
I'm 64 now, and I still remember that taste.
What a splendid idea.
I've known about rosemary roast potatoes (and rosemary yorkshire pudding) and marmite mushrooms for a while, but not marmite roast potatoes.