But wait - I fixed it !
Well technically I found my problem which was not the client - building that locally did work.
I hadn't appreciated that the admins at work had turned on client UUID verification, so the profile I had used wouldn't work on a different machine 🤦♂️🤦 (it's already behind SSO so this was unexpected).
Luckily I had a second profile, without the UUID requirement and using that works !! The SSO browser page launches.
It would have been nice to see some logging though.