Re the invoicing question, you would probably have to give them a written quotation or estimate before they agreed to your doing the work, and you would need their office postal/reguistered address in order to do that. You need to find out before you start the job where you should invoice and how they like their invoices made out. You wouldn't want to start work for a new customer without knowing how and when they would pay you! You would need to draft up some terms and conditions, to be on teh safe side too, eg payment expected within 30 days etc