I have had a reply from the £300 guy.
£300 becomes £400 if the earthing needs sorted.
How much is 6mtrs of 10mm Earth, a box of clips and 2x earth clamps.. ~
You forgot to add all his years of training and experience so he knows how to properly size the wire, and run it in neatly in the proper place and connect to the correct bit of pipe work, and ensure the resistance of the finished job is low enough.
Oh and his time to go to the wholesalers for the cable, and his diesel and motor, PL, and PI insurance, road tax, income tax, vat, accountant fees, holiday pay, sick pay, and I could go on and on.
The guys running a business and needs to earn a living after paying out all his business expenses. The likely hood is that yours will be the only job he'll be able to do for the day. He will be lucky to earn much more than £120 of actual profit from you. It's not exactly a brilliant wage.