Unistrut really worked well ive not used it before but like it. £11.58 per length, from FH Brundel(thanks for the link, its a great site). Defo opened up a few options for future projects.
All cuts were coated twice with the cold galv spray. The pattern one which the solar chap had made for me to copy i rebuilt with all zinc nuts and bolts adding washers as well. He had used A2 and i prefer zinc as strut is plated with that.
String line used to set out and the hypotenuse across all four is dead flat. Really happy with the way it came and thanks for assistance/pointers.