Ok the ending is dissapointing, very dissapointing.

It took my 15 hours to complete and i'm sure i have missed out on some side missions and i haven't done the sewer missions. I want to go back and do the these. However i saved my game half way through the last mission so what i want to know is can i leave half way through the last mission and continue doing side missions? Or will i need to start all over again and make sure i complete everything before accepting the final mission?

Cheers.

You can go back and play everything you haven't done. Go and don't forget, take your time, explore and apreciate the beauty of the work id did !