Grant, an estate mill was a source of revenue for the laird, the tenants were "thirled" to that mill and nowhere else and a portion of the meal ground went to the miller. He sold this meal and thus paid his rent. He also had the power to destroy querns where he found them and the tenants were also required to provide so many days free labour a year towards the upkeep of the mill & lades, much resented apparently. Interesting that your folk were from the Beauly area, Forbes of Culloden who organised the resistance of the Hanoverian clans in the '45 had a wee estate called Bunchrew just west of Inverness and was Lovat's neighbour. Lovat played a double game and turned his coat so many times he couldn't tell out from in. He ended up on the block and possibly this Captain John Forbes who factored the estates was one of the Culloden family. Culloden's losses in the Rebellion, (Culloden House was looted of its fine furniture by redcoat officers who shipped it to London), were compensated in part by a license to distill whisky.
Gairloch's estates were not forfeit as Mackenzie of Gairloch kept clear of the rebellion unlike Cromarty, Scatwell & Fairburn for example. The Mackenzie's originate on the west coast but most lairds also had estates in the east, where they lived in the Winter. If one of those families died in the west their coffin was shouldered by relays of hundreds of their tenants over the hills, to Beauly for burial, much lubricated by whisky it must be said!
When Forbes of Culloden's mother died the grief-stricken funeral party arrived at the kirkyard minus the coffin, the Forbes's were renowned for their fondness for a hauf.
By 1800 the old feudal milling exactions had been abolished but they probably never existed in Wester Ross anyhow. My own ancestors were also millers and a good book on the subject is "The Scottish Country Miller, 1700-1900," by Enid Gauldie, pub' John Donald.
The only Forbes mentioned in the book is Captain John Forbes, it's just strange that at the time he's writing to the Government for permission to build mills in 1755, millers of that name appear in Wester Ross shortly afterwards.
Bests,
Skoosh.
Found a reference to a meal mill at Bunchrew
http://bunchrewhousehotel.com/history/the-forbes-clan/Also a reference in the Inverness records of an Alexander Forbes, the miller at Castlehill Mill, June 3, 1603.