Fixed ... :D
The bulbs from Super Cheep work but I had to cut the original bulb out of the bayonet holder -it was soldered in.
Anyway it truned out not to be the bulb, but a dry joint on the clock PCB itself. I ended up pulling apart the whole clock. Word of warning : don't do this unless you absolutely have to. It is a bit of a pain to re-assemble.
On mine, the green colour actually comes from a green filter mounted directly behind the LCD screen - there was no sock over the bulb.
Oh ..and the credit card idea worked a treat tks....

