I am making a diorama with German soldiers, the set came with a decal set for the German helmets but I am unclear what to use. My research online suggests that by 1943 German helmets largely went insignia free. Which insignia would be appropriate for a late Normandy earlier holland German unit

No insignia would be your best bet for that period.

Helmet insignia was stopped in 1940 and in theory should have been removed but inevitably some soldiers didn't do it so you will still see late war photos with troops wearing helmet decals. By 1943 helmets were manufactured in plain field grey. Seems strange that OKW stopped the wearing of the tricolur first then followed by the eagle yet factories still manufactured them with decals sometimes. Not like the Germans to be inconsistent. Both Wehrmacht and Waffen SS insignia can be seen late war but it's more the exception rather than the rule. By that stage of the war it was quite uncommon to see them.

Sorry forgot to answer your original question. By Normandy/Holland helmets wouldn't have insignia but if someone had served pre 1940 or even up to say late 1941 they would probably still have helmet decals. It depends if you're modelling veteran troops or newer troops. I'd say if you're doing a dio you could have decals on one or two with the majority plain. If you're doing Normandy it was quite common to see helmets repainted in dark yellow, green and red brown in the field to camouflage it and whilst wet, sand would be sprinkled over it so it stuck to the wet paint. When it dried it eliminated the shine of the helmet as shape, shine, silhouette gives away a helmet at a distance

Personally I would say for the time period you are interested in, it would be no. However I expect all military stores of every army in the world operate on the same basis, you got what you were issued with, even old stock.

If you do use any decals on an older issue helmet, chip and scratch the decal so it doesn't look too new.