Hacker & Shanwick...
Apologies..Lakeside Aviation was the name of the outfit I was trying to remember, NOT Highland. NewAir was the Biggin service and Lakeside had been the Inverness-Liverpool schedule that I was trying to recollect.
I think Gill currently operate a scheduled Mail service most nights between NCL and LPL but that is I believe the only schedule just now. If Gill survive their present troubles and the SD3-60's disappear, then it may become part of the ATR operation.