Hello again all,
So... After various support calls to Apple, receipt of a new machine, escalation to 2nd line support etc, I have been informed that Apple engineering have acknowledged the problem and will address it in a software update. I'm assuming this will be an update to OS X as I have also been able to recreate the issue in QuickTime in the interim (and of course people are seeing the issue with Garage Band etc)
I have formally asked Apple for an email confirmation of the situation but been politely refused on the grounds that Apple doesn't send out this kind of information because it can find it's way onto the Internet and, I suppose, be presented in a certain context. To a degree I understand their position about this, however, given that users on this thread first started posting in February I have mixed feelings about just how long a solution will be in coming.
I explained to the technician that I now have a £2.4K machine that I can't use the external speakers on - in other words - this problem isn't an esoteric fault that only a handful of power users may stumble upon. It's pretty basic stuff and they need to get a move on and fix it.
My request for formal acknowledgement of the problem was made in order that I have a hard copy of documents at my disposal should I need to exerise my consumer rights over the coming 12 months in the event the issue is not resolved. On the basis that Apple won't formally provide this information I have requested via email, and am now in receipt of, the fixed address for Apple's lawyers in Europe. I will need to write a letter to them to explain the situation and have them confirm to me that my intention (as another poster here quite rightly suggested) is either for a full refund of money, or receipt of a brand new machine within the 12 month period from the initial purchase if, for whatever reason, they have not solved the problem.
In terms of Apple's 1st and 2nd line support department I can't fault them. They've been very helpful and quick to respond.
I guess I now have to purchase some external speakers. Not happy, but my hand is forced - at least for 12 months.
I'll post back here after the next update to OS X (10.15.7)?? with a report on whether or not the issue my side is resolved.