So to close this off, I managed to fix the iMac but there were two separate problems.
# The first problem, LED 2,3 were green, but not the others, was '''solved by reseating the RAM.''' I then tested using the display before seating the parts back in place, but when I put the display back after everything was screwed in, LEDs 1,2,3,5 lit up, there was no backlight but I could use an external monitor and see the apple logo in the display when I shone my camera flashlight on it.
# This became an Odyssey. LEDs 1,2,3,5 lit up when the Mac was powered, and LED 2 when power was connected but the machine was off. I replaced the display, logic board and power supply at the expected cost but nothing worked. A final suggestion from [link|https://www.thebookyard.com|The Bookyard] suggested '''changing the eDP cable''', as it was the only common part left to the new and old components. And hey presto, the backlight worked. The say that the backlight failure and the eDP cable have been implicated before and in my case that was the cause.
# The LED diagnostics and in which order they start up didn’t match with what I observed or expected. The Bookyard said they hadn’t seen my sequence before, esp LED 2 being on instead of LED 1. Food for thought - and these guys were extremely helpful.
I hope this helps someone else - and thanks to all those who suggested fixes.