I knew that we had a similar item somewhere, and having just turned the house upside down, I've found it. Eureka.
It is the same as Roy's - the pattern is identical. It has a very clear lozenge in the middle of the inside and (using the online lozenge translator) I have the following:
This glass design (Parcel No.2) was registered on the 6th of June 1876 by:
Whittingham & Percival, Manchester.
Double checking in Jenny Thompson's "Identification of English Pressed Glass" shows the Pendelton Flint Glass Works, Whit Lane, Pendleton (as TC notes above).
We have always believed this item to be a salt.