Atlantic GIant (adn they'll be a heavily selected variant of AG) can be a bugger to get started.... I always give it 3" pots in my thermostatic controlled propagator set to 28 degrees.... previously I've bunged them in with the other outdoor cucurbits I grow and they don't germinate in the rather cooler 8w fixed ones, they just rot, meanwhile around them, marketmore cucumbers, Winter Festival squash, Becky pumpkins and all manner of courgettes and gherkins are racing away....