I wish you every success Michael, and would be interested to learn if you ever managed to get anywhere with your quest.
I have a number of albums from their international and their poetry output and have been searching for years for three MacColl, Seeger albums I know were recorded but never issued - in vain
Someone announced he had compiled all of International and British folk music albums on this forum some time go
He had a website - he might be able to help - I'm sure a search would trace him
best wishes
Jim Carroll