Really really dumb question
 

....where EXACTLY is the ECU ground?

I always thought it was the grounding strip below the screen wash bottle (on a UK model).

Am I right? Or is it within the cabin/footwell nearer the ECU?

There is more than one ground I believe. There are at least 4-5 ground wires coming out of the ECU. On my 97 there is a ground just below and to the right of the TB. Looks like a black tab with a nut on it. Also, I think m.net's garage section *might* have a list of them (for cleaning up the connections).