Wrote to Chamberlain about this and got a reply back which said that
I'm supposed to use a "40w rough service edison screw bulb". First
reaction was "WTF?". Second reaction - where do I get these from.

http://www.tlc-direct.co.uk/Products/LARS40ES.html

The door opener will be subjected to much vibration, which massively
shortens the life of the bulb. Flashing and flickering do not affect bulb
life. I wouldn't be surprised if flickering extended bulb life because the
filament never reaches full temperature...

Christian.