sorry wot is a ARB?

ANTI ROLL BAR,

the Octy does have a rear one per say (just a bit of metal welded into the rear beam). Fit an aftermarket one to this and as stated above it DOES make a noticeable difference to the car's handling (in a positive kind of way).

I would recommend Whiteline rr solid bar £130-150 ish and is adjustable

you can have it understeer or neutral (stable) or lift off over steer if your feeling adventurist. Had mine a couple of years and cant complain