Success!
The purchase of a EU region infotainment/satnav unit has resolved the issue. I bought the unit for £90 from
https://www.volvosaabbreakers.com/ in Nottingham.
I could fit this part because I configured the car to accept the foreign unit through the d5t5 dashboard.
So - all that I'm waiting for is the registration documentation.
I've spent an extra £320 on top of the breakdown that I put on the thread at the beginning - this includes the SatNav unit, some extra software I didn't realise that I needed and the time of a guy call Matt (
https://vvde.info/wp/) who's been helping me with the d5t5 software and region change.
It's been a great experience, and I hope that I've shed some light on the process and provided some good contacts for anyone wanting to go down the same route.