Had the modem replaced yesterday - absolutely no loss of connectivity on the way home now and no longer needing to tether to my phone.
The saving going via the SOS function as the attack point was around £150 which was considerable IMO - a third off - as I am out of warranty.
Modem could be replaced yourself - assuming a level of competency and has a part cost of around £80+VAT - toolbox/coding was not required according to the tech I spoke to, it's a separate plugin card that goes on the MCU - bit like a PC expansion card.
Anecdotally, while speaking with the engineer in the coffee/waiting room on pickup another customer piped up and asked if that would fix his issue too - Model Y with intermittent connectivity issues being his fault - so this issue is still very much out there beyond us "nerds" who gather on the internet to bitch about it