Oh sorry @Sporky. I should have read your post more carefully. I was mainly going off your picture. I can see now it is the potting mix on the leaf! That's good you don't have sooty mould.
Losing a couple of leaves is not a big problem. If you keep on losing them though it could be due to a number of different reasons.
First thing to check is the drainage holes in the bottom of the pot. Make sure they are not blocked as drainage crucial for lemon trees. And you shouldn't be watering much (if at all) at this time of year.
Another culprit could be wind. Is it in a very exposed position? It will be happier if it has a bit of shelter from strong wind.
And have you been feeding it? Make sure you don't feed it too much, especially at this time of year and nothing too strong. If you use something designed for trees planted in the ground, such as granules, then the tree can become stressed and drop its leaves.