After a few trials and tribulations, I think I've cracked it.
Initial searches gave me two options:
"Something akin to LMS grey" or a few cases stating Humbrol 27 as a starting point (given a white wash after to lighten the colour).
If I were to go strictly by Nigel Digby's depiction, GCR grey is slightly darker than Midland grey, and I wanted to keep to this as I want variation within the wagon fleet (rather than painting everything LMS grey, as my English Oilfields wagon was also painted in this colour!).
With that in mind I purchased a tin of Humbrol 27 and got this result:
Better, but still a touch too dark, almost reminding me of an LNWR grey (which proves why this is used as a starting point and lightened thereafter).
I then experimented mixing in a lighter grey, eventually settling on a half-and-half mix. This is what I've ended up with:
Much better! And yet, still meeting the criteria of being slightly darker than Midland/LMS grey (I used the EOL wagon to verify that!).
Happy with my achievements, I've painted the underframe black and will continue building the wagon tomorrow by fitting the floor and building up the wood colour inside.
- James