From a Google search:
I thought it was the W&L that was a GWR extension....
As for soft or hard C in Celtic, that was prompted by a reference in Sean O'Casey's autobiography, where he satirises the S pronunciation used by some at the time (he being a Nationalist and later James Connolly's general secretary). It's hard to work out why, as we're nearly all happy with the soft C in cycle or ceramic, and Celt being from Keltoi.
But to be serious. I'm working on a drawing in original form with a view to maybe one day making a model, and what I can't work out from drawings or photographs is which side MGWR drivers were. Long shot, any photo of the backplate?