In the 1970's I was involved with the complete renewal of the track drainage in the Standedge Tunnel, and also the engineering track inspections throughout all 3 miles, 66 yards of it.
I was told it was pronounced 'Stannage' or 'Stan'edge'. I do not recall ever hearing it referred to as 'Stand-edge'.
As an aside, within the old single line tunnels, there was a structure called 'The Cathedral'. This was a cast iron archway situated midway between Yorkshire and Lancashire. I have no reason to believe it is not still there.
Donald Fairweather, limpingdog@talk21. com