First of all check out the neighbours planning permission on the local council planning portal and check the conditions imposed.
Look at the drawings to see if they've made the extension larger than allowed. It's my understanding that you can no longer build up to the boundary line, to allow for maintenance access. If that is the case, speak to planning.
Next contact building control and express your concerns regarding the soffit/facsia board. Depending on what you discover you should really speak to your neighbours, as they may be unaware of the problems caused by their builders.
If you still have no joy, perhaps go to Citizens Advice, or a solicitor. Good luck!