If you are absolutely certain that the photo was taken in 1985 and not 1987 or later, then it is looking like this was a indeed a test run. The Cornish Skippers came up north around 1987. Newton Heath and Neville had them at first, but I think that most had settled at Heaton by 1990, leaving 142027 at Newton Heath.
I stand to be corrected, but I seem to recall reading in Motive Power Monthly in late 1990, that 142020 was the final 142 in a different livery to the GMPTE orange units, to lose it's SCG gearboxes. I believe that it was converted to Voith at the same time as 142001-005. (142010 had been done in September and 142006-009/11-14 were completed by late March 1991).
At this time, Heaton 142s did not operate in the North West and I don't think that they operated around Leeds either. At the time Neville Hill had an allocation of around 27 142s and these regularly appeared on North west services however.
