Roger is correct in the make-up of the plumb pudding igniters Bruce., These 'rifles' were not done on the hoof so to speak but converted to a set design standard with a shortened butt stock bolt and threaded into the chamber oif the tube. We had one complete mortar and a second 'rifle' at Warminster and while I didn't take much notice of them, they were clearly reasonably/obviously well thought out, done to a set pattern and worked.
Apparently, the weapon collection didn't have a plumb pudding bomb thinggy intil they were clearing away some of the area to ascertain the viability of building of the third London Airport at Maplin Sands. (nope....... it's still not even in the design stage............) There a huge cache of puddings in reasonably good condition was unearthed so we got a couple. I used to wonder how they projected in their sort-of flight path although 'flight path' and 'accuracy' might be taking such words to extreme. They were probably as aerodynamic as a piano