Thank you for the information Silver Moose!
Volvo's new 11 point check does include Road Force Balancing. But unfortunately we live 70 miles from the nearest available facility (as confirmed by your link, many thanks) and my dealer said that was too far away! Everything else was completed including tyres off again, inspected, re aligned and balanced, also included on the list were some engine mapping resets as well. They then compared it to other cars on their forecourt and said ours was a little worse on some road surfaces and handed it back to us to check out.
Initial impressions were a favourable improvement, but this could have been just road temperatures (hot Easter weekend) as next drive at a typical UK 9C showed the issue was still there, if a little better than the last attempt they had.
I had already researched the Road Force balancing before taking the car in as I have a friend who owned an early Audi Q3, which did respond to this quite well, but only for 2-3 thousand miles before his similar vibrations came back again. Hence I did not see this as a desirable solution and did not push for this too much with the dealership. Admittedly it would be good to know if this could cure the resonance/thrumming, but for me Volvo would have to agree to get this done for the life of my ownership of the car and at no cost to me.
As you say, the best wheel balance a car can have, should be at the factory, the second being the Hunters machine and finally the least effective, a bog standard balance at the tyre shop (which has been absolutely perfect for all the other 25 or so cars I have owned). So as you say, either there is a problem with the chassis significantly magnifying a slight issue, or there is an underlying problem on just our individual car or even both?
Next step they tell me (when the dealership gets us an XC60 loan car and they have the right level mechanic available too) is to swap out wheels, tyre brands etc, with new cars they have in the show room etc, and then do side by side comparisons. The engineer did say drive shaft replacement is down on the list of actions from Volvo as well.
As I have said before from the outset, the issue does feel tyre related. Every time we get the car back, the fault is a variation on what we had last time. Different frequency, sometimes there are resonance peaks at certain speeds, sometimes it is constant over the full speed range, sometimes you can feel vibrations through your seat. Also generally speaking (as it is never exactly the same, but is always there) it is more apparent on colder days before the tyres have warmed through.
On a more positive note, I can confirm that my neighbour (now an E-Pace owner), also managed to get a full refund from the dealership ( as per AndyP ) after they could not resolve the same fault on his XC40 T3 FWD. Although there were other factors involved with his experience with the dealership too.
XC40 | First Edition | D4 AWD | Bursting Blue | 19" Continental Premium Contact 6 tyres | Ordered 14/03/18 | Delivered 11th October 2018 | UK.