Genius Rob! (Have you done this before?).

I know a head gardener on a country estate and have just come out of a meeting with him. He recommends drilling a small hole into the base of the tree with a cordless drill. Flood Glyphosate (Roundup or Clinic) into the hole. Then use a prepared wood filler, coloured to match the tree, to close off the hole. It will take a while to kill them, in which time any saw dust left from the drill will have disappeared.
He thinks it is very improtant to give off a fiendish laugh when the operation is complete but i'm going to save any smugness for the time being.

Thanks all.