with regards to the will situation, if i have a letter (from first solicitor who gave my brother and i advice), stating on it that my brother agrees to adding my son to the will - if i show this to our solicitor dealing with the estate, would it help my situation as it is a legal document?
It's not your brother that has to agree - it's all the other beneficiaries. If some of them are minors, they cannot make that decision themselves and can (when they get to be 18) sue the person who made it for them, unless that person is the Probate Court.