You've rated the answer, but I haven't really given you one as yet. Let me do that now, and if you would like more information about it, just let me know, and I can give that for you.
okay, so a right to light issue is difficult to run, but very possible. What you would need to show is that you had a right to receive light through a defined apateure (i.e. window). This is done usually by showing that you have received light through it for at least 20 years, and that it has become a "prescriptive right to light".
Then, you need to make sure there is nothing in your property deeds that prevent you acquiring a right to light (in fact, there may be an express right in the agreement).
If you can show the right to light and a serious interference with it, then you should be able to bring an action to prevent the light being blocked out.
If this is something you think you would want to proceed with then I would encourage you to see a property litigation solicitor that specialises in such types of claim as they can be a little tricky.
Hope this is useful.