OK, thanks for all the advice, I'll take the light off sale for now.
This light seems to be quite complicated, and not the straightforward switch on/off.
I've tried two 0.5W 12V bulbs which don't appear to work, they check out OK on a multimeter.
So I've tried a different LED and that does seem to switch off after a delay, it comes on with the interior light which has a shorter delay...
So I have a working boot light, sort of!
I fitted a genuine LED Fiat Lancia boot light to my 2004 Dynamic, and that stays on when the hatch is closed for about 5 mins. I don't worry about it as the current draw is next to nothing ( Search posts on here )
So I've tried a different LED and that does seem to switch off after a delay, it comes on with the interior light which has a shorter delay...
So I have a working boot light, sort of!