Bilt4me - great minds think alike! Reading through other threads on here about similar issues, it seemed to me that he may be the one who can shed more light on this, and I've already messaged him, so hopefully, with a nudge from yourself as well, he'll be along in due course to give his thoughts.
sbman has already proved to be a font of useful info, and already I know stuff I didn't know a few days ago, so hopefully, between us, we're heading towards a solution.
It occurred to me, after some of sbman's comments, to try something I hadn't tried before, and maybe the results do indeed point towards this being the immobiliser causing my issue. See what you guys think: I disconnected the battery and left it for long enough for everything to be cleared. Now my thinking was that, if it is the immobiliser, and this is essentially a computer - and a pretty old one at that - then, like other computers, it may need some time to 'boot up' after power is first applied. So, with the help of an assistant who was ready with the key at the ignition barrel, I connected the battery and immediately had my assistant insert the key and go straight to cranking. My theory was that, just maybe it would run for a bit longer under these circumstances, as the computers would be busy getting their acts together as they had only just been powered up. And guess what? It ran for about five seconds, which is a couple of seconds longer than it's ran for since this problem appeared. To be sure this wasn't a fluke, I went through the same procedure three times in total, and every time, it fired immediately and ran for around five seconds each time.
Now bearing in mind that previously, attempts to start it, even the first attempt of the day (but when the battery was already connected) have often resulted in it not firing at all, and when it would randomly start, it was never for more than about three seconds. So, does this support the immobiliser theory? I can't think of any other reason why it would run for a little longer than normal if it's started the instant the battery is connected, unless there is some truth in my aforementioned theory.
Discuss...