This handy Photovoltaic calculator is a great research tool for 'What If?' calculations.
For example, a flat roof is very good over a whole year, if you accept worse performance in winter, but months of excellent performance in summer. (You would also have to consider dust laden rain falling on the surface leaving patterns, and so on, but theoretically it works, and the panels can be tilted to the south. If South Facing only, then undoubtedly, 36º is the best angle for Latitude of 53º north. But I was surprised to find that for an east facing roof, a 10º pitch is far better than a normal pitch because it gets better south and west sun. (makes me think of all those shallow pitch portal frame sheds in industrial estates). Wall mounted panels give acceptable performance with a more even spread of power delivery through the year, but falling off alarmingly if you haven't got a good south angle.
I have a friend who is due to build a house and it is going through Planning. She would like it to be green, but its present design doesn't have any South Facing roof. Even if the payback argument says not to instal this year, what about in ten yrs time when higher electricity prices and cheaper panels make the payback very good? I contend that if we are the slightest bit green thinking, we all MUST build with enough south-facing roof to accommodate at least 4kW of capacity, for future occupants to install.... (but the payback argument is better than it will be for the next 5 yrs).