Along a section of wall I have (a) painted plastered brickwork on the right hand side and (b) painted drywall on the left hand side. The drywall flexes very slightly relative to the brick, so whenever I paint over the transition, it inevitably cracks sooner or later. Is there any simple solution to this please?
I think you'd have to support the drywall to make sure it does not flex. That would involve demolition, adding support that is evidently lacking now, and replacement; and may not be worth the effort.
Placing a molding over the crack to conceal it would be another approach.
Cutting it out to 1/4 inch wide and filling it with caulking as an expansion joint is a third option, but probably less appealing. You'd likely want to hide it, in which case you dont need to bother, you can hide the crack with molding, as mentioned just above.
You need to put fiberglass wallpaper on those walls. Here in The Netherlands it's called "renovation wallpaper" and it exists specifically for these cases where you have transitions in wall type.
My house is full of drywall-concrete and drywall-brick transitions (renovated old office building), and I covered those walls with a thin, smooth fiberglass wallpaper before painting. The wallpaper has enough flexibility to handle the transitions while not cracking the paint. I have no cracks.
You can get this stuff in various thickness and with or without patterns. In your case, go with a smooth variant of medium thickness. I would recommend not to use the super-thin variant, as it is a pain to put on. You need to make sure the seams of the wallpaper are not on the crack and you need to do the entire wall, at least to a point where you don't see the edge of the wallpaper anymore, e.g. a corner.
You need a flexible transition at that intersection of different materials or you will continue to see cracks due to different expansion characteristics of the different materials.
Use a flexible paintable caulk all along that seam, let it dry then paint it to match. This is a very common solution for this common problem.