Following my original post I had several long sessions with Apple technical support - who were very helpful. However the result wasn’t the one I’d hoped for.
it appears that my 2017 MacBook Pro should not have been capable of supporting the two external monitors that it had been doing - a limitation that was clear in the tech specs for the MBP. However - for reasons unknown - Monterey seemed to do something with the resolutions that allowed this to happen. Neither the first level nor senior tech support people could explain or replicate this. And whatever (albeit useful) aberration that had existed with Monterey was no longer being replicated with Ventura.
My choice therefore was to try and go back to Monterey or update my laptop (which I had planned to do in 2023 anyway). Looking at the current range of MacBooks it became clear that there are only a couple of models that have the required specs to run two external monitors so I’ve recently upgraded to a 14inch M1 Pro - which is running beautifully. See photo.
Overall it’s been an odd experience. I received excellent customer service from the tech support people I spent time with. But it remains a mystery as to why my old MBP was doing something that was beyond its specified capability.
