Hmmm....I think I've found the answer to my question - no, can't connect a phone simultaneously with wi-fi hotspot and CarPlay. Bugger.
https://www.slashgear.com/wireless-c...cord-16527444/
I had a little more of a play earlier; the article above confirms what I saw and didn't manage to achieve; some strange device names added (my CarPlay device name is "spenny_b iphone"....my hotspot profile is "spenny-b-iphone", note the hyphens instead of underscore and a space between spenny_b and iphone.)
At this point, the feature has very limited use for me, the only time possibly being where I desperately need to hop online with a laptop whilst on a business trip, and my mobile cell coverage is marginal for pairing to the laptop - I may then have a strong cell signal using the cars hotspot....maybe.....certainly not worth paying £10/20/30 a month on the offchance!
ETA - what I hadn't realised (or rather, forgotten since quickly pairing my phone) is that the CarPlay connection also uses Wi-Fi in addition to Bluetooth. I had assumed it was just a BT connection, the same as my previous non-Carplay car. This is my first outing with CarPlay. Of course, now I know it needs Wi-Fi as well as BT, it's entirely logical that you can't have 2x Wi-Fi connections simultaneously.
Oh well, my daughter can use it with her phone on tomorrows road-trip I guess. #wontbesubscribingafterfreetrial