Hello!

[Ernest]

Then I'll check whether there is a later monitor I should be burning into the EPROM......

That's a great start Dean,
To change or not to change the Boot ROM depends on the current revision of Boot ROM, if the Mainboard is a Premium or a Standard etc.
There are boot disk images available in the Repository once you have applied for access and agreed to the terms and conditions.

You may like to start a new Thread in the Microbee Hardware section to go from here, rather than in the Introductions section.

PS As the 256TC uses an internal switchmode power supply, taking the DC input and producing the required regulated 5V for internal distribution, the input (within reason) is not a big deal for short term testing. Long term the supply will run a little hotter with 12V as against 10V however it is very well designed with a very decent sized heat sink. 

Ernest