when is the best time to harvest my pumpkins and as i have a large one on the plant will it stop the other flowers turning into pumpkins

Depends a little on variety. If this is one of those enormous pumpkins, if you remove it unripe, you won't have time for new ones to form and ripen.
Large pumpkins are better and will store only if fully ripened, so leave it on the plant until just before the first frosts, to dry and ripen. Then you can store it in a shed or garage until you want to eat it.
If you want lots of pumpkins to eat during the summer and autumn, I'd recommend a small variety like Jack be Little or Sweet Lightning, the new T and M hybrid. We find these taste waxy and are very pleasant unripe, and when left to mature have flowery starchy flesh. Each plant will produce a large number of small fruit which you can remove regularly and still have sufficient to keep for winter.