Well of course it is! Thank you for reminding me!
Making progress but still not all the way. Placing a copy of MPASMX with its name changed to MPASMWIN.exe and adding the following line to the UMCbuild.ini file made THAT error go away
Code:
MPASM=C:\Program Files (x86)\Microchip\MPLABX\v3.55\mpasmx
It then complained it couldn't find the MPLAB device path so I added the following:
Code:
DB=C:\Program Files (x86)\Microchip\MPLABX\v3.55\mpasmx
(Because, just as with the assembler, that's where all the device specific assembler .inc files are located - which I assumed was what it's looking for...)

But now it says No device file found for "18F452"

Anyone have any ideas?

/Henrik.