So, I couldn't sleep in the heat and got up to make myself some toast and marmite. While I was waiting for the toast to pop I found myself reading the back of the marmite jar. Best before March 2012. It's a large jar. I've been eating it a few times a month for... (based on the brand saying they put 18 months on their 'best befores') 14 years! It tastes absolutely fine. There is around 1cm left in the bottom of the jar, no mould. Frankly I'm impressed at it's longevity.
Now to decide, do I buy a new jar that might be less concentrated, or see it through to the end? 🤔