Get it done at an independant Renaultsport specialist and it'll be more like £450 to £550, inc. auxillary belt too, and will be done by people equally if not more familiar with the RS models than a Renault dealer themselves. As above £780 does sound very familiar so probably the standard Renault price for a 172/182.
BTM performance
Rentech
K-Tec Racing
TDF (I assume they do it)
Birchdown (think they're called this)
etc