So I've resolved the issue - with a little assistance from a very helpful guy at Dell support: I tried flashing BIOS back to 1.3.7 - no luck. Flashed to 2.1.0 again - same. Dell support suggested restoring BIOS settings, so I did this (to BIOS default) and lo, the system booted back into Ubuntu! For the record, what has changed in BIOS settings was the boot order. Previously I had: "Linux-Firmware-Update\fwup64.efi" listed first. Now boot order shows: "UEFI:THNSN51T02DUK NVMe TOSHIBA 1024GB Partition 1" Anyhow, this gets my machine to boot with BIOS 2.1.0.
Can someone from Dell please provide some detail on what the 2.1.0 BIOS update enhances/fixes? Has anyone successfully applied it to the 9360? Also, I've noticed that Ubuntu Software app no longer picks up BIOS updates (I have to check manually). I guess this may be due to the GUID changing as a result of BIOS 1.2.3 (LVFS lists the XPS13 9360 twice). Does anyone know if this is the cause and if so how to fix this so Software can pick up BIOS updates for the new GUID (5ffdbc0d-f340-441c-a803-8439c8c0ae10)?  Thanks
BIOS 2.1.0 update