I've just come off the phone to them, very nice lady at their end but she certainly wasn't trying to sell anything.
Basically infilling with the root balls may not work as the established roots of the other privet will take over if the new plants aren't cared for. (I expected bonemeal and mycorrhizal fungi to help with that) only difference is the 15l root ball will be more stable in it's infancy than the 4l ones but no more likely to take.
She said to cut it back harsh around now before the birds nest then keep cutting as often as possible. Again I asked if maybe I should buy their bonemeal or something to help the new hedge, she said I could do and it wouldn't hurt but not to bother.
If I really felt the need to infill with anything then best bet would be to get a load of small bare roots and let them mesh them selves in...
Not really the answer I was hoping for but I guess it saves me £400 and gives me optimism that maybe the hedge will survive. So plan thus far; cut the hedge back tomorrow, take it all down to around 7-8' clear out as much debris as I can to help get the air and sunlight through and take that conifer back. Come May I will order maybe 10 of the 5' 4l root balls, cut back the rest of the conifer and other junk and plant the new privet along with a load of fertiliser and go from there.
Sound like a plan?