Maybe silver or golden thyme?

Just thought - if you use gravel it will be more difficult to pick up the bits when you clip the hedge! Are you expecting the hedge to get wider as it matures? If so perhaps some largeish pebbles might answer and they could easily be removed later if necessary. I'd definitely not use membrane in that space.

Many thanks for the suggestions, the hedging is about 7 years old and has grown very little i had thought it would fill the space.

Yes you wold have thought so. Never mind it looks nice as it is.