The 904e is a less powerful bike, more mainstream like the eZee Sprint etc., so I wouldn't be confident about it's performance on those extreme Bristol hills. Wisper are a new company here, but are reported to give good service. Possibly they have an owner in that area who could oblige you with a test on those hills.
Only the Forte/Forza and Powabyke have the very high power motors.
Wisper are bringing out a 904se model which could be more powerful, but it's likely to be more of a Torq competitor, possiobly geared for speed rather than additional hill climbing. Of course until it's launched and on the road, we won't know it's abilities, but it's been delayed for a while now and should have been out months ago. Therefore very much an unknown quantity.
.