I think that it must depend on tradition of what ever core your with. I heard a few things said a couple of years a go.....let me think.....Right, I believe that it was not a spoken term, ie if you were Lcpl then you would be called Cpl (in spoken terms) but on paper you were a Lcpl. I will have to do some digging.....
LCpl was a rank given to the army by Queen Victoria, it is higher than an AB in the RN and higher than an SAC in the RAF, the other 2 services have no equivalent, when being addressed a LCpl should be known as Cpl by all ranks.
Rubbish! the RSM is referred to as Mr ...... as a matter of courtesy by very very junior Officers, there after it just depends on how well you know him, if well then Mr ........... if not then RSM.
I will caveat that by saying Corps or Bn traditions will supercede this.