Thanks everyone - these are certainly the ones! First time that I've seen such a photo - and I've been collecting SMJ images for nigh on 40 years.
There are a few shots around of the full wagons in the exchange sidings awaiting a locomotive to take away, and just one I think of a 350hp diesel shunter that appears to be shunting, but might just have been up to the quarries.
Interesting to speculate whether the double heading was very common? I've seen one other pic of a single Cl26 between Blisworth and Northampton on the LNW branch, and assumed that that was the usual consist, but it will no doubt have depended on how many wagons there were for disposal.
One of my contacts who was a signalbox lad at Blisworth in the early sixties says that the regular train was 8E55 Blisworth-Frodingham at 12.45 in his day - obviously the two 26s would have had to run round/reverse to get away towards Northampton - and presumably they had worked in with empties earlier. The other pic between Blisworth and Northampton suggests that it might have been a Northampton job rather than Rugby, but I don't know the full state of play back then.
Incidentally - is there a precise date for these shots - the quarries closed in 1967.
Are there any more interesting SMJ shots out there like this....keep 'em coming please!
Thanks again folks