Lots of the structures on this section seem to be similar to the standard mk1 structures but with minor differences. Lots of the cantilevers seem to be standard MK1 masts except slightly taller as shown by Clive Mortimore: https://www.rmweb.co.uk/community/uploads/monthly_2019_06/25161000_ScRmast9.png.629b9c78f6fa5e937de3061149b5928f.png . Most of the portal and 2 track cantilever structures look as if they are made from Circular Hollow Sections which are bent for the diagonal members and welded together: https://www.rmweb.co.uk/community/uploads/monthly_03_2015/post-16423-0-45625200-1427837001.png . Most of the power supply structures seem to be separate to the main masts, these look to be mostly a pair of towers with some wires dangling between them but only on this section.
There also seem to be lots of portals with what looks like fixed termination points, these look like they are mostly made out of H sections. In the areas where a station was, you can get very long spans such as round Blisworth, possibly due to other sidings and platforms in existence whilst the OHLE was being installed. These tend to be heavier (n brass type trusses), but have some of the lighter trusses and some Pratt truss types. Round the station area on your layout, you might want to have a truss spanning from the viewers side of the platform to the far side of the back platform.
For construction, Eileens Emporium do metal sections and wires, n brass do insulators and portals and shapeways will probably do alternative insulators.
Best of Luck,