OK guys, I apologise for seeming to flog this subject to death but it is my main day-job to work out how we can film St Elmo's! I'm really grateful to everyone who has helped me so far.
Some people seem to have never, or rarely, come across St Elmo's whilst for others it's a very regular occurence whilst flying, and it seems to me that there are two reasons for this. One is flying different routes (plus some people don't have the option of avoiding the rain/hail clouds that seem to be best at creating it), and the other is the type of aircraft. St Elmo's tends to form on sharp edges and points, and it seems that refuelling rods right above the cockpit give the best view - hence my question about which aircraft (other than the Nimrod and the Hercules C-130, which I know about) also have this arrangement. I do apologise for my lack of knowledge on this, and thank you to everyone who has helped me.
Alex