In reply to Only a hill:
I thought I would add to this thread for posterity, as I've now phoned the stores in question and obtained some hard facts!
Shiel Bridge: the chap said that they usually stock meths, but if critical for success it's advisable to ring up two weeks in advance. He was very obliging and said he would be happy to keep a bottle aside for me.
Kinlochewe: same situation as Shiel Bridge. Shop usually stocks it but owner happy to set a bottle aside if asked in advance.
Ullapool: didn't bother phoning anywhere, as I'm 100% sure of finding it somewhere in town.
Kinlochbervie: I phoned the Spar shop, but they don't stock meths. Apparently the nearby garage has it in stock.