Hi Gary,
How much the tip is should be reflected by the service you receive. If you are happy with it by all means give the ghillie a bottle of malt , he will appreciate it no doubt !
However, and this is the really hard bit , if the service is crap , the manner inappropriate and you are made to feel in any way uncomfortable
then state clearly why , and don't give them anything , with dignity.
I suspect that you will be buying more than one bottle though , Tweed Ghillies are generally pretty damn good.
Paul.