Get a load of cuttings of Lonicera Natidia, early October, and stick them in
6 inches or more, along the base of the existing hedge every 6 inches,
single or double row, they will root over the winter then grow up through
the hedge, not all will survive but I think you will be surprised at the
number that take.
I did this 3 years ago and the hedge now looks as if it is 50 % lonicera.
and if you can scrounge the cuttings then its also a very cheap solution

--
David Hill
Abacus nurseries
www.abacus-nurseries.co.uk