any preservation company will give you a guarantee that will comply with the bank/building society scrutiny,and even the so called big boys will suggest it is an insurance backed 1 in case they go out of business.
if you want to go down that route of lifting to save money then do,cant for the life of me see the point of them trying to carry out a treatment if its in situ.industry knowledge that they wont get to the underside of a 1st floor as usually there is plasterboard attached to them,so that leaves 3 sides to treat,and your quote so far has said they want to leave the fibreglass there?so that leaves just the top to treat.AND there willing to guarantee that are they??mmmm,that in itself would start alarm bells ringing for me.