Also some info here...
http://www.rics.org/Practiceareas/Property/Rig htstolight/Right%20of%20light.html
Do you know the people who lived there before or be able to get in contact with them in order to see if they would be happy enough to provide a statutory declaration about their 15 years plus to make up the 20 years?
If you don't know who they are or how to contact them post another question in law and I'll try and help further.
You could try any papers you had when you bought if you have your title deeds or any copies or anything like the contract or transfer on sale or old copies of their mortgage deed or searches in the deeds.