The only reference I have for Air Ecosse and a One-Eleven is indeed G-AXMU, leased from BIA from 23 Jan 84 to 6 Feb 84 - just a couple of weeks. Probably just covering an unserviceability.
It then went off to Airways International Cymru in their first year for the summer, running from Cardiff for Red Dragon Travel, when it was more likely to be a "Holiday Jet".