".........Would it grow up the middle and would it struggle for light and
food with the existing growing again in the spring? Existing is maybe 3
feet deep or
more. ........"

Yes and Yes, but if you cut your existing hedge back when it has been in
leaf for a few weeks, say Sometime in April then it will be a couple of
months till it gets thick again and taking light from the Lonicera, yes it
will grow leggy but you can cut it back in oct or as soon as the leaves are
off the thorn and it will thicken well 2nd year, and where it is on the
surface then it will thicken with clipping.
--
David Hill
Abacus nurseries
www.abacus-nurseries.co.uk