Wow David Alpha - many thanks indeed for such a rapid response.
I'm still reading like crazy. I saw how to boot into FEL mode, but I didn't realise the VID/PID might change as a consequence, and I'm wary of "doing anything" without double checking everything first, in case I make matters worse.
I've tried the Keysight I/O Libraries suite - it fires up fine on my pc, but fails to detect the DSO2D15, and I can't tell whether that is a pc / driver issue (I've tried both the default IV Foundation driver, and the Zadig installed WinUSB one), or the DSO being unresponsive at the other end of the USB cable. Which is why I suspect I'll end up with the Platform-tools way. The DSO does complete the boot process, but then freezes with all the on screen settings as they were when the problem first appeared.
I'll try the FEL boot mode and see what happens (I'll have to tell my anti-virus to unquarantine DSOflash.exe first, but I know how to do that). I'm a bit wary because sadly I hadn't done any backups before I had this problem - didn't even realise I could or should - newbie error! :-( and there seems to be a chance I'll lose the serial number etc (which you have provided a utility to restore :-) thanks!, and some calibration setting (which may not be recoverable, at least at my current state of knowledge)
When/if I get things working again, I'll be sure to back up everything in sight, and install the factory reset daemon!