As you can see from the Googlemaps photo (taken about 4.30pm sometime in 2007), there is North-South ridge, but there is a clear 10º bias towards the south if we use the East roof.
So the West roof has a 20º disadvantage. But it is illuminated in the afternoons, when the air temperature is hotter, and the skies less hazy, although afternoons tend to be cloudier.
Access is governed by the Velux Rooflights which only face east. PV panels work better in cold air conditions, so the East face is better, because we know the roof on the west gets infernally hot in the afternoons.
It is tidier for maintenance and installation to have all on the East. We would not want to have anything above the glass conservatory incase of a dropped spanner by someone doing maintenance.
Following some of the decisions on the use of Plastic pipes instead of Solar panels (described above), we are likely to mount those on the south wall (despite the nearby tree) and maximise the PV on the East Roof.