I use black reptile heat lamps not 100 watt bulbs.
Do you have a thermometer in with the ducklings? Can you raise the lamp you have and shine it to the side so the little ones can move to where they are comfortable?
For week 1 ducklings need to be at 90F 32C; week 2 at 85F 30C, week 3 at 80F...
I raise ducklings in large plastic storage boxes: the sort that have lids that clip on, although I don't use the lid. I use chicken wire across the top and folded down at the sides of the box. I am not sure what you call them in the UK: we call them plastic totes. The chicken wire should keep...