In all the photos of sufficient quality to be able to tell, the fishplate, check-rail and cross tie nuts appear to be hexagonal. Other examples of their use on FB rails at Tanfield and MSLR at Brockford make me confident that the assumption is plausible. Also..... Scale Hardware only do hexagon nuts!
Code 100/7 roughly equates to 11cm, so I reckon your garden rail is about Code 55/7 Richard.
To be honest, cast brass fishplates never occurred to me. The WMR examples do a good job as functional joiners of standard length track panels, with jigged, pre-drilled rails. As with many castings, they benefitted from a bit of fettling, and their intricacy and hardness made this very time consuming. I'd had enough after 20 or so sets, so I have the utmost admiration for John and the team for completing the whole layout!
Also, as Colin suggests, it became an absorbing project to solve all the manufacturing issues, with pretty basic technical skills and equipment. Now, having all the components to hand, and an efficient bench set up, I look forward to about a fortnight of pretty relaxed assembly work.
P.S.
Sopwith is rather bored with trackwork, and currently has his eye on the 'Amusing Hobby' Neubaufarzeug PzKf.V1 Kit.