Hi Stu,
I wonder if there is something amiss with your MEP mini-route installation. I've got MEP installed as a mini-route on both my C:\ and D:\ drives and they both work as intended with virtually any amount of OpenRails folders in the Engine and Wag folders. Further, the Dreary Day DMU scenario opens up in the activity editor just fine. See pix below:
MEP_MiniRoute.jpg DrearyDay.jpg
Are the modified engine files located in an OpenRails folder within the MEP-BRCW104 folder (see pic below). If this doesn't solve the problem, I wonder if a deletion and re-install of the mini-route might help? Just guessing here, I hasten to add.
BRCW104.jpg
Cheers,
David