Should have added to my post that I realise I probably could not stop the development going ahead on a "right to light" basis as others have said (that is based on the law as it currently stands). However I would simply inform them that if they go ahead then at some future point I would be suing them for loss of income as I already have several years worth of PV generation data to refer back to and sunshine data for the same period (in case they try to argue that it hasn't been a very sunny year, hence my PV generation is low).

The problem with anything 'legal' in my opinion is the way that the profession homes in on the tiny parts.
Some things are implied, some are mandatory, language gets distorted, there is no difference between 'will' and 'must'.
You may possibly be able to argue with your installer. They would have had to do a prediction, if they did not specifically mention that a new building could reduce performance then you are in with a chance as that would have been their negligence. Would be a very long shot though.

These are some of the problems with not fully investigating all possibilities when purchasing something.
I suspect that you have to take this one on the chin (re the reduced output), so look at what you can do to help yourself i.e. shading control of modules, different angles, relocating them. Probably cheaper to do that than fight in a court.