For everyone who commented to help, thank you all, it has now been sorted.
I got in touch with the shop it came from and asked if they could find out the password for it. They told me they had tried already but had no success getting through to those who donated it. I was given the option to go and swap that laptop for a different one of pretty much the same spec but with a few small upgrades over the last and generally much nicer to use. I took the offer, went and checked first that it had no passwords on it anywhere, took it home and had got Arch installing straight away.
I never though that a password blocking one setting in the Bios would cause so much hassle, its something I'll keep in mind for any PC's I buy used in the future.