whwoz, try really soaking the seeds. If they're sprinkled already on your potting mix, water them til it runs away. and do that repeatedly. Celery has germination inhibitors in the seeds (this is why parsley is so slow too, apparently). They have to be dissipated by water before they germinate.
Celery seeds do want light to germinate, so sprinkle them on top, press them in but don't cover.
You probably aren't using a heat mat where you are, and that's good, let them cool down at night as they would naturally when the sun goes down, the natural day/night cycle seems to help too. If it's hot at night... try sticking them in the fridge at night? I dunno, could be interesting.
Celery are very slow growing so just be patient, keep em wet, and feed em a liquid fert if you want to see them grow a bit.