It can be done in situ,it's not that difficult as long as nothing's rusty or seized.
I think it took me about 15 to 20 mins.
I used a flexible screwdriver with a Torx bit and had to loosen the power steering fluid reservoir and fusebox to gain access to the 6 bolts if I remember correctly.
I tested to see if it was likely to be the ECU by plugging in a loose known good ECU when the light was on .
It took about a minute to do and the light went off,but came back on when the original ECU was plugged back in.