1. Will the windfarm have any physical impact on the archaeology?
An access track will overlie a stretch of Roman road. Otherwise no know sites will be affected.

2. Exactly how close is the windfarm and how big is the henge and its environs?
Normangill is quite a small henge - there's a visit record somewhere on this site. The poor thing has a road running through it which may be used for access to the windfarm site. The closest turbines will be a few hundred metres away on the nearest hill ridges. The "environs" of the henge are best described as the valley in which it stands so the turbines will ring the periphery of the henge's field of view. Normangill cairn is situated on one of the hill ridges and will have a 120m high turbine about 50m away.

3. How far from the people to that need the power is the windfarm to be placed?
The windfarm will feed into the national grid. Its not a local initiative, its a power station with about 175 120m high turbines, covering an area almost 20km long by up to 10km wide. If you were driving down the M74 with your foot on the floor it would take 10 minutes to get past it.
However, we should give the developers for being honest here. From what I can tell, its more normal to get permission for a much smaller site, then "discover"a need to expand it.