Thankyou for your responses everybody. Clearly there is still much good feeling around for these two airlines.
In the early 1970s I was pre-school and regular trips to Leeds Bradford (Yeadon in those days) were a way to keep me and my siblings occupied. It was mainly Northeast and Dan-Air aircraft back then so I always found the BIA Herald visits a bit exciting (and the Aer Lingus "Dublin jets"). Then one year, probably around '77, BIA stopped visiting and the Isle of Man flights were no more. By this time Air Anglia had established a base at Yeadon so maybe BIA felt unwanted (!) although I don't remember Anglia flying to IOM, just the likes of Edinburgh and Norwich (the easterly bias I've already alluded to). And Amsterdam from 1976. I don't remember seeing BIA at Yeadon again until early 1979 on training flights. The company might even have resumed an IOM schedule. I can recall seeing their Heralds at Manchester and Exeter, hence the apparent westerly operation.
What I'm fairly certain I do remember is an Air Anglia timetable with a notice detailing the company's B&C ownership and pictures of an F28 and a BIA 1-11. So the two companies' common ownership, by 1979 at least, seems assured but it appears the east/west operations split wasn't by design.