It's been a long time but I thought I would give an update - have finally gotten the car back on the road! Loads of fun - especially given the hot weather
Ended up going with the original ECU (defeated immobiliser and had the EVAP/SAI/downstream lambda removed) and a chargecooler
A few things to still work out. Coolant temperature is one - does anyone run an up-rated radiator? I'm fairly sure I'm running the original rover rad. Seeing coolant temps up at 110-115C if you really push it on a hot day. I imagine this is coming close to the limit of where you want to be. Discovered, however, that if I remove the number plate from infront of the air intake and stick it to the bonnet you can get it down by 10C which is a good start!
Second issue is boost pressure, which is proving baffling. Boost is stuck on the actuator pressure (max 7psi). Logging the ECU with VCDS shows requested boost at 14psi, but the ECU doesn't control the N75 to match - the duty cycle is stuck at 0%. Engine will rev all the way to 6k, so doesn't feel like limp mode and it's not throwing any fault codes. ECU can also switch the N75 with the VCDS output test, so it doesn't appear to be wiring issue, or problem with the valve. System has also been pressure tested and no issues - it's a bit confusing!