Thanks guys for the comments and great info DR20T!
As Mick said, if the biu, cluster, keys and ecu aren't coded together, then it won't work. Swapping just a cluster is pointless as it will reject the key and throw on the immobiliser light. Thankfully, I had access to a subaru dealer who were sick of seeing my car in the end, but they were nice enough to reprogram the cluster, ecu, biu and keys to match without charge. One thing they won't do, and i have no idea why is reprogram old keys to match. Having said that, there's a few documents online that tell you how to do this but having two new keys, i haven't bothered trying.
This issue was considered unsolvable by the guys at subaru and it even came to the point they offered me a trade in solution and they would just send the car off to the auctions as buyer beware. They were quite supportive, but sadly at the same time, they were a bit rough when going through what they did (exposed wires, broken clips on panels, etc).
The loom i got was from an 07 post face lift and having a my08, I was really surprised seeing how in just 6 months difference in build date, how much it had changed in sections.
The CANBUS system relies on twisted pair wires for communication in sections and i sadly now know that the twists are a certain distance and you can't have it non twisted any further than 8cm from the end of the plug. But Mick is absolutely right, the whole CANBUS system was created to reduce the number of wires in the car, and it really does talk to every system and sensor.
It feels like that more bolts and clips went back in the car when putting it all back together and my greatest fear was hearing rattles from panels. I'm glad to report that the previous rattles i used to have are now gone and i'm relieved.
Thanks again guys, and lets hope no one needs to ask me questions about wiring diagrams.
Back to tyre shopping!
Brian