folks, thanks for the suggestions.
(assuming you'd read the comments on the flickr page) I know ... but you see I'd just bought these new darkslides. They were a more ancient breed of Lisco's than I'd ever seen. The last straw was that this bloody sheet didn't want to come out of the holder. They don't have the recess ground into the alloy partition sheet in the middle to get your fingernail under and it had simply stuck to the base.
but I've got a strategy now ...