ok firstly it is de-gassed

secondly i am replacing the heater/aircon distribution box and to pull it out i need to disconnect the aircon tubing near the firewall, any idea how this is done??

well im just guessing u have a e ser if so
it's easy at the tx valve ( pipes on firwall that go into small alloy bock )
just remove the 8 or 10 mm bolt in the centre of the 2 pipes the the 2 hoses will pull out then looking in to the tx , in the center of it u will see 2, 6 mm hex bolts undo them then the block will pul off