your bf is lucky to get a plot of land - they can be hard to come by!
I would recommend speaking to Buildstore - they are the experts in this type of thing. they can offer mortgages for these types of self-build projects. I belive they are offering up to 85% borrowing for land purchase and build costs, but they did do 95% before the credit crunch. Not sure what the % would be for just build costs though. There arent many lenders who will lend for a self build, but BUildstore work with the ones that do. They can give you money upfront for your build phases, so you get your cash before you start each phase, handy for buying materials and paying your builders! not sure where you live but they have a massive exhibition centre just outside swindon too - i visited a while ago and its fantastic, especially if you are a bit of a novice!