Somewhere I have an old print photo of this occurrence that I took from below the viaduct and another taken from the station the following morning. From memory the unit involved was 141104, I can't recall the 156. I was at Huddersfield Polytechnic (now University) at that time and lived in Fitzwilliam Street, about 300 metres south of the Viaduct where it had occurred.
In those years I would have had a fair number trips on 141s (including some on 104 prior to this) they were common on all the routes to and from Huddersfield in my years there.
I remember seeing the 156 and 141 on the viaduct walking home in the late evening. It was dark, but street lights lit up the main roads close by. As they were stopped in an odd place, I walked up to the viaduct to get a closer look (from below) and it was only there that it became apparent that they had collided.
The damage didn't look too serious and I concluded the speed was clearly quite low. I assumed the accident resulted from a SPAD, but don't remember reading about a report, although RAIL magazine might well have commented on one.
In a sense it is a shame that the thread excludes those in Iran. There is very little information that I have found on line about where they were put to work, although there are a few fleeting pictures and a reference to some or all being out of service by 2005, but still extant. There is a more recent picture of one dumped in sidings on this thread
http://www.railforums.co.uk/showthread.php?t=76653 about them. It would be interesting to know if any UK Railway Enthusiasts managed to get some Iranian track in for 141s!