I've just bought an STI vers5 Type R, it has an apexi cat-back exhaust fitted, I was wondering what I need to make the exhaust "straight through" so to speak??

 

V5 type r's only have a single cat on the downpipe and a resonator on the centre section. If you have a cat back system then it's probably that your original centre has been replaced to a straight through section. If you want to remove the downpipe for a decat version I would recommend a remap, especially on type r's, or you will be prone to det
I've left the original downpipe on mine for the MOT and went for a Revolution 3" straight through system