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Before I take the whole thing apart, is there a trick to getting the cable out? It appears to be held quite firmly in, when I try and push it out it just pushes the internal round.
No tricks, probably just jammed in it's housing, and yes, it'll probably need taking apart to push it out. A pair of pointed pliers will probably do the job of gripping the inner cable at close quarters, try to avoid using anything sharp/pointy as it may lead to bloodshed as you'll be holding it very closely.
Sounds like you speak from experience of sharp/pointy objects and bloodshed whilst taking levers apart. Will get dismantling this afternoon.
Dismantling a SRAM shifter is dead easy, however rebuilding it is a task akin to attempting to nail a blancmange to the ceiling.
Do not under any circumstance attempt to dismantle the lower shifter assembly...
Also watch out for the slidey gear indicator thingy being in the right place. It's held in situ by a flimsy plastic cover which is easily dislodged.