Hi,I've not had chance to reply before now, but I have been going through ps of the S&M looking for POWs too.
Breeze Ltd has been mentioned, as has Sydney Yates and Lilleshall.
One of the reasons for the continued existence of the S&M was the quarry at Criggion, their wagons, predominantly drop-down 3 plank - not unlike the Hornby BQC offering. Other wagons carried Ceiriog Granite, Granomac and Granoplast liveries. These excited the S&M just south of the Abbey station.
Other wagons seen on the S&M include Brentnall & Cleland, Wm Cory, Cory Brothers & Co, and John Potts of Macclesfield.
Hope this is some help,
martin