Hi Sue.
If you go with builders mortgage, you will need bank account to pay for mortgage,and normally they like you to have there own bank account, plus you will need bank to pay for service charges,water, electric, etc, the NIE, completion, you can do yourself, if you are over at time of completion, if not you will need solicitor, to act on your behalf, then you will need to give him/her power of attorney, it sound complicated, but is not, just your preference, if you go with solicitor, then they will charge, sounds about right for fees,if using solicitor.
Regards
Michael and Yvonne