I ordered one for testing, as shipping is free, with a view to ordering some more from the guy if the first worked. It arrived yesterday, after a long wait (cheap postage from China is evidently cheap for a reason). So tonight i soldered it onto the old plug and popped it in an eMate.
It makes an excellent fit and all seems well: the battery is recognized and charging in a way the old one just would not.
Mind you, I think I'll test the smoke detectors before going to bed tonight....