Who are the men David sees as he's sitting and eating?

 The leading man appeared regal, had red hair, and was accompanied by a lawyer, servant, and man of the Sheriff.

I saw four travellers come into view. The way was in this part so rough and narrow that they came single and led their horses by the reins. The first was a great, red-headed gentleman, of an imperious and flushed face, who carried his hat in his hand and fanned himself, for he was in a breathing heat. The second, by his decent black garb and white wig, I correctly took to be a lawyer. The third was a servant, and wore some part of his clothes in tartan, which showed that his master was of a Highland family, and either an outlaw or else in singular good odour with the Government, since the wearing of tartan was against the Act. If I had been better versed in these things, I would have known the tartan to be of the Argyle (or Campbell) colours. This servant had a good-sized portmanteau strapped on his horse, and a net of lemons (to brew punch with) hanging at the saddle-bow; as was often enough the custom with luxurious travellers in that part of the country.

As for the fourth, who brought up the tail, I had seen his like before, and knew him at once to be a sheriff’s officer.