We found the Maille shop almost accidentally - it's located on Piccadilly, London, between Fortnum & Masons and the Ritz, opposite the Royal Academy in a corner shop on Piccadilly Arcade. Website here Maille
They make mustards with wine instead of vinegar, giving the mustards a rather potent but seriously delicious tastes. We tried a small 108g jar of the Cognac mustard costing £5.50 which is gorgeous - slightly sweet, but very tasty
Back label with ingredients
Lid
This can be purchased online here Cognac Mustard for £5.50 per 108g jar
There is a very large range of Maille mustards both in the small jars and a slightly larger range of big jars (at least double the size of the small ones) costing £12 a jar. Expensive but I suspect a worthwhile treat for the foodie connoisseur!