I’ve found detailed statistics on factors affecting solar output published by the Microgeneration Certification Scheme (MCS) in their 2012 Guide to the Installation of Photovoltaic Systems. This confirms “inclination, orientation and shading are the three main site factors that influence the performance of a PV system” (page 58); although, as they report, latitude is also significant, with the south having more solar irradiation than the north (see picture on page 56).
The MCS guide includes solar generation tables for zones throughout the UK (see pages 60-61 and page 101), which details how solar output varies with roof slope and orientation. [UPDATE: I’ve now prepared a version of this table showing how output varies for our zone – see section on Factors affecting solar output in Wivey (part-way down costs and benefits page).]
The peak roof angle for our zone is 38 degrees. Output is less than 1% lower at 30 degrees and only 2% lower at 25 degrees. For steeper roofs, output falls by only 0.5% at 45 degrees.
Orientation has a greater effect. Panels facing SE or SW have an output 5% lower than those facing South and those facing West or East have outputs that are 18% lower. This compares with my initial rough estimates of being 10% and 20% down respectively, which I have now updated.
For those wanting a simple guide to how solar output varies for a range of orientations and roof slopes, the MCS guide has a good indicative chart (figure 17) at the top of page 55, which shows variations for a mid UK point (the Wivey position will only vary slightly from this).