To save you the bother of typing it, I’ll help you out,
TL:DR
Still not much further forward, despite reading a gazillion reviews of both contenders 
I’ve decided the Sony A9 II body is over budget for me, however the original A9 isn’t. I see that they share the same sensor AND processor. It appears that the main differences are ergonomics, connectivity (I’m not overly bothered about that) slightly faster burst speed (not a deal breaker) and 1/2 stop increase in OIS.
As one review stated -
Compared to many other camera upgrades, these tweaks might seem incremental, but then the original A9 was already an excellent shooter. Sony could have easily gotten away with making the camera body more ergonomic while leaving the innards untouched.
With a (discounted) price difference of over £1k I can see how I could be tempted by the original A9 + 200-600 zoom and a 2x teleconverter. Total weight would be 2.9kg and best price seems to be ~£4.1k
Interestingly, the rival Olympus system I’m considering is in a similar boat. The OM-D E-M1 Mark III share much with the original choice - OM-D-M1X and is around £500 cheaper.
Kit weight (E-M1 iii + 300mm f4 + 2x tele) is 1.9kg and best price ~£3.1k
Where does that leave me?
£1k more and 1kg extra weight for the Sony. BUT full frame sensor, higher ISO capability and slightly faster (anecdotally more reliable) autofocus.
In my mind, the Olympus system had the edge (weight, lens speed, price) until I started looking at the alternative bodies mentioned above. Now it is looking more like 50/50 or even the Sony edging slightly ahead.
I suppose the clincher is going to be how they feel in the hand. Location (and Covid) makes this problematic, but the only alternative is to buy on the basis of reviews… 
