Hi Pete. It's been fascinating following developments from the original boxfile. Two thoughts:
1) There's no problem having the BG in the middle formation, most BGs were fitted with cages. Moreso, I've walked through several 'uncaged' BG's as well in the past. MBM have connectors for Mk1s as well: http://www.mbmodelrailwayproducts.co.uk/n-gauge-products
2) You acn double head with your 37s - just run them around one at a time. You could use the excuse that a particular type of heat was required for the stock, ie by keeping either the steam 37/0 or eth 37/4 coupled inside to the coaches.
I hope 'll be able to catch up with Kyle in the flesh at some point!