Engine Overheating (04 octavia Estate)

Hi all - had a recent problem with the engine overheating on my Octavia Estate (04). Cutting a long story short, the problem was in the harness to the small fan - the wiring had burnt out and it had fused.

This was replaced and the small fan now kicks in to cool the engine but we have been unable to get the large fan to kick in.

Does anyone know when the large fan should work? Is it just the engine isn't getting hot enough or is there another fault somewhere I need to be looking for.