We are going to replace the carpet in our hall stairs and landing which
are a fairly large area.

Existing carpet is hessian? backed on Uniroyal Tredaire Oben underlay
which to my untutored eyes looks to be in good condition.

I suspect most of the carpet retailers make most of their profits out of
underlay. Anyone knowledgeable about the need to replace underlay?

TIA for any advice.