Thanks Christine

I've gone cross-eyed looking for a mark of any description, and the only "mark" I can come up with is on the top of the out-turned rim. It could of course just be my imagination

trying to make something of what could be just a ripple in the glass, but I think it looks too symmetrical to be just a random manufacturing flaw :huh: It's very faint, but looks like a hill shape with what could be a letter A beneath. This may or may not be the letters H and A for Hazel Atlas :huh:, but looking through images in the previous links and on the web generally, the hen doesn't match the Hazel Atlas ones shown.
I googled "Shirley Smith+hen" as you suggested, and found an excellent Convention download "Hen on a Nest - Studying a Glass Collection, by Shirley Smith - Presentation materials." This includes a really good section on how to properly observe the features when trying to identify hens on baskets/nests. So using the observations hints given by Shirley, I can add some info which might help in the ID:-
Comb - curly.
Tail - split/divided. Scalloped edge.
Feathering - realistic.
Head - cocked to one side.
Eyes - flat but goggle-eyed
Eggs (which I didn't even notice!!!!

) Four in total at the front - two on each side, the front one being smaller. Two at the back - one on each side at the base of the tail.
Nest/basket - basket weave (picket fence?), which continues on the base.
Scalloped out-turned rim.
Colour: Base - milk glass becoming opalescent at the scalloped edge. Hen - grey marbled.
Overall form - the top fits well on the base so may be not a "marriage."
I think it is older than those made by Mosser, which according to Ivo's A-Z would date it to 1964 at the earliest. It has wear and tear on the base and some nibbles and wear elsewhere. I got the impression that it was fairly old and the seller thought so too. I got it at a knockdown price, impulsively buying it because the collectables/junk shop was sadly closing down. One less place to mooch and root about in

I might just pop back and see what else takes me fancy