SOLVED (for me anyway)
I worked my way through the sample page for the screen and at one point thought my screen was broken as no words displayed, although I could get the light on. The problem was a real novice mistake! See if this is you too...
The early examples on the page work as soon as they are sent to the pico, and the light stays on. The later examples put the code with in onInit, but with no call to the onInit function! So when you send the code to the board, you have to manually type onInit() on the left hand side of of the screen otherwise it won't run.
I'm embarrassed to say that this took me about 2 hours to work out - but hopefully writing it down here will help out some others too.
(There is an inconsistency in examples, sometime they add a call to onInit() at the bottom of the script, other times they don't, so be on the lookout for it.)