Besides being a mortar round, I can't say for sure. I also inherited a number of rounds from an uncle that I could not identify (they were repainted so they would not be mistaken for live rounds, but this was never passed on to me when I inherited them).
I went to the British Ordinance Collector's Network and posted on their forum to help identify them. Good bunch of guys and really helped me out.
BOCN Let us know what you find out!
Jamie