If the polythene is left on too long now the seeds have appeared, they may get too leggy or scorched from the direct light passing through polythene. Therefore I thought I would raised it slightly.
To do this, I simply place a large flower pot in the center and place the polythene over the top like a tent. To help prevent direct light scorching the new seedlings, I put some fleece under the polythene. I did this for all 5 Carrot drums.