Possibly a leaf hopper Eupteryx melissae It is a pest for commercial growers but they never seem to do my sage any real harm.

Many thanks Pimpernel. My plant looks so healthy I wouldn't like to see it die off now. Magreen

I wonder if the Piece of card smeared with Vaseline would work to get rid of them..? Basically take a large bit of card smear it all over with Vaseline...hold it over the blighters and when they jump they stick to the card...you can use the card over and over. As I eat my sage I would not use chemicals.

..or try yellow sticky flypaper, available from garden centres. I have heard that you can vacuum them up too. Hold the nozzle above the plant and tap it to make the flies jump up.

I will have a go with the sticky card first - it almost sounds like a fun thing to do. I'll let you know how it works. Failing that, the hoover test comes next. Thanks everyone.