Does the Chess Valley affect solar potential?

It can, mostly for east-facing roofs. PVGIS terrain data for central Chesham shows the skyline at 10.7 degrees from south-east round to north-east, against 1.5 degrees due south. For a 35-degree roof that cuts modelled yearly output by about 1.1% facing south, 3.5% facing east and nothing facing west. The HP5 3HA postcode point in Chesham North ward loses less (0.6% facing south, 1.0% facing east), so estimates should be run for your address. The model uses terrain data at about 90 metres, so trees and neighbouring buildings must be checked on site.