A common problem with Listers is that the cooling air, having passed through the fins, is not expelled out of the engine room but is able to recycle back round into the cooling system. In other words, the air must be ducted outboard straight from the cooling jacket, not just discharged into the engine area. Also ther must be an adequate inlet for cool fresh air for the fan to drag into the cooling jacket.
Another thing to check is that the cooling fins are clear; you will see in the Workshop manual a special tool, a purpose-shaped rake, for dragging through the fins to shift gunge and debris.
Plenty more things to suggest, but check those first.