For those of you who have been following/commenting/helping with my plight you might like to know that I've solved the problem for now - my power supply (an old transformer) was good enough to power the LED but wasn't giving enough to power the MJLJ itself. A 9V battery has made my evening!!
THANK GOODNESS!!
Massive idiot alert - problem solved!!
Moderators: JeffC, rdoherty, stieg, brentp