Aidan of EvoEnergy has pointed me to a marvellous site on the European Commission Joint Research Centre site, enabling calculation for performance of Photovoltaic panels at any angle or azimuth anywhere in Europe or Africa. This takes into account effects due to temperature, reflection, system efficiency etc. It is good for roofs, walls, and even for motorised panels. It will calculate the optimum slope for a given azimuth at this latitude (the best azimuth is obviously from 10 to -10 around south.)
For my Tall Building project I can see that it is very useful for calculating PV panels on Facades. (click on images to enlarge). On the calculator map you can zoom down to a specific town or even street.