Well said, Eddie ................ the "right to light" is not as simple as most people think. A solicitor is definitely recommended.
Legalities aside............. on a practical note........ assuming the project goes ahead, talk to the Planning Officer about the height. 9 metres is way more than necessary for two stories. In fact, negotiate! The Developer will want neighbours on his side if possible. Unlikely, I admit ........... but 7 metres is fairly normal for two stories if there is a gable end. There doesn't have to be ......... a "hipped" end (slopey bit of roof just like the front and rear slopes) will reduce wall height to around 5 metres....... making a lot of difference to your light.
Challenge the whole proposal NOW ........... in writing. Get on record. Take legal advice........... better still, employ a Planning Consultant. They are more clued up than lawyers.