It's the cutting - frequently!

Keeping them a little smaller at the top than at the base will keep them full to the base. Feeding them occasionally helps, too, though they don't need much. Good compost plus a little blood or fish meal, in spring, would probably work best.

Thank you both - feeding and cutting it is then.

I have to say it won't be a chore with my new cordless hedgecutters - its like the texas chainsaw massacre at the moment - I love it, I can see why the male fraternity love their power tools !