Hello Michael,
I did have access to a full set of ME's, but don't any longer, but your example is no doubt based on the E L Pearce loco described in ME in 1901 in what was the first 'sort of' construction series of a miniature loco.
Phelps of Treherbert doesn't ring any bells with me, despite having a long association with South Wales.
Cheers,
Julian