Thanks guys - yes I do use a ridged metal roller, but even the pros get voids sometimes. My cardboard plug probably didn't help matters there, either
The temperature thing is a bit of a bugger. My garage is unheated but I'd been using the resin successfully for small jobs like the door hinge pockets when it was colder than it was on Monday afternoon.
It's possible that the large flat area meant that the heat from the exothermic reaction didn't stay in the panel for long enough for it to cure properly as I also noticed that it was still tacky some 20 hours after it was laid up, despite having 5% catalyst mixed in. The small jobs i'd done a week earlier were all rock solid and dry to the touch within 12 hours.
What I do have now, though, is a nice rigid plug that can be used to create a really decent floor, once I've fixed the issues outlined above.