So while the new mixer and vapouriser on my 4.0 have seriously improved things, it would appear that both my lambda sensors are knackered! Neither of them are switching although judging by the performance things like the MPG on the trip computer dropping from 18mpg to 14mpg, the one that drives the gas system was probably okay the other night.

With the laptop plugged into the gas system I can see it's pegged fully lean with the stepper trying to open as far as it can go. Normally at all speeds (even at idle) it hovers right around the 90 mark with the odd excursion up or down under acceleration and lifting off. Would it be reasonably safe to just set the stepper values to lock it to 90 until I get some lambda sensors delivered? I figure that would make it work pretty much the same as a fixed power valve.

Yup. You can set the default under the optional configurations tab but you will also need to set the in and out of idle upper and lower steps all to zero or it'll still adjust to default + upper limit.