Continuing a theme of small but perfectly formed pottery, this little mustard pot certainly fits into the category.
It's by Poole Pottery and has a delicate hand-painted decoration in lime-green and charcoal-grey. I would say that the pot dates from the 1950s and I think the pattern name is Slits. The mustard pot is part of a cruet set and would have had a matching salt, pepper and stand to go with it.
There's lots more Poole Pottery on Rob's blog, here Rob's Poole Pottery Blog