After nearly giving up earlier today, I stuck with it the job is done and the ABS problem seems to be fixed.
I undid the four subframe bolts and dropped the engine as far as it wanted to go but the booster still wouldn't come out. Someone on another forum mentioned the upper engine mount, when I removed it the engine would drop a heap more. The process for removing the booster went something like this:
You'll need 13mm deep socket, 4 car stands, a decent jack, garage creeper, 10mm, 13mm, 15mm, 18mm, 1/2 and 3/8 drive sockets and spanners. Torx bits/screwdrivers. Long small screwdriver, pliers, breaker bar, pincers, 15mm hose clamp.
Remove the engine cover.
Move the driver seat fully rearward.
Disconnect the battery.
Remove interior trim panel above driver's knees, 2 torx screws.
The booster input shaft is connected to the brake pedal by a pin that is retained by a small clip, remove the clip with a small screw driver and a pair of pliers. Free the booster shaft from the brake pedal.
In the engine bay, unplug the RHS O2 sensor, the plug is right next to the end of the crank case vent chamber thing at the top of the engine
Remove the black plastic crank case vent chamber cover thing from the top of the engine, there's about 15 torx bolts holding it down.
Remove the vacuum line from the booster. The hose clamp is a challenge. I used a pair of pincers to open it up. A new hose clamp will be needed.
Remove the booster heat shield, 2 x 13mm nuts. Don't drop them
Remove the next two 13mm nuts that secure the master cylinder. It isn't necessary to open the hydraulic system. Remove the hydraulic lines form the clip on the firewall. The master can be placed on top of the engine still connected to the hydraulic lines.
Remove the brake pedal position sensor from the booster. It's a cylinder shaped sensor about 30mm diameter and protrudes 60mm from the booster. Unplug it and remove the retaining ring. It will pull straight out but there might be a bit of residual vacuum to fight.
Unplug the sensor on the other side of the booster. The plug has clips on both top and bottom which need to be released before the plug will come out. Its a tough one. This is the sensor that was broken in my car, it is integral to the booster and is the cause of this entire debacle.
Remove the upper engine mount, there's 5 x 15mm bolts to undo. Note exactly how it was orientated. The arm part can be installed either way up, installing it the wrong way causes a pile of engine vibration in the cabin.
Jack up the car using the very front of the subframe as the jack point. Support the car on 2 stands positioned behind the subframe using a small length of wood to distribute the load. Remove the jack. I'm not sure if this is an actual jacking point but I didn't damage my car. You need to support the car in a way that does not involve using the subframe because it has to be free to drop down.
Remove the splash tray, there's 6 x 10mm bolts
Loosen all 4 of the 18mm subframe bolts. They seem to be held in with red threadlocker so expect a fight. Remove the 4 x 13mm bolts from the rear subframe mount.
Loosen the steering rack. I'm not sure if this is necessary. The rack is attached to the subframe and it will be dropping so I loosened it off a bit to allow it to move slightly.
Get the jack under the front of the subframe and completely remove the front two 18mm bolts but put them back in with about 4 full turns so they are almost all the way out. Drop the jack so the almost undone bolts hold the subframe up
Place the jack under the rear of the subframe and remove the two rear 18mm bolts making sure that the jack is there to support the subframe. Using the other two stands, position them under the rear of the subframe so you can still get at the bolts. Drop the subframe off the jack onto the stands, there should be about 100mm drop.
Get the jack back to the front of the subframe, undo the last of those front 18mm bolts and support the front of the subframe on the jack. Lower the jack so there is about 100mm drop. Leave the jack there.
Back inside the car, using a deep 13mm socket, unscrew the 4 booster nuts. These are oval nuts so they'll fight you too.
You should now be able to remove the booster.
Hope that helps someone, I was not able to locate any info on brake booster swaps on RHD XC90s
Engine bay reassembled
Engine bay reassembled, with the engine cover removed. My car has had a weird problem with a lot of the corrugated loom tubing. It just crumbles to dust. In this picture I've replaced most of it and also used cloth tape.
Proof! The booster is out!
This picture shows how far the sub frame LHS (passenger side in Australia) needs to drop
This picture shows how far he subframe RHS (driver's side in Australia) needs to drop.
