Basically, you're both right, I didn't actually see these myself, so the only resource I had was a video called "British Rail today" and quite an interesting video it is as it talks about how Britains Railways changed after the privatisation occurred, but yes these were used on the WCML around the 80s and 90s era, very soon after the Railway Privatisation when Stagecoach partly owned services on the WCML.
Real life phots below:
Also, '87015', looking at your photos, noticed you went to Nene Valley Railway Diesel gala . A great day wasn't it, 56057 - MY LORDZ!!!!!