Unless its specifically designed to go in that make of CU, it is unlikly to fit the live rail, you really need it in a separate mini-cu box at one end or the other of the exsting box with cable in and out (and wired like any other L-N double pole switch - look at the incomer for inspiration, but don't touch)
Consumer unints that have been designed to take an RCD as part of a split load conversion, have a gap in the live bar, and a length of something suitably bosky to bring the neutral to where it is needed. A single width RCBO usually has a double decker contact that puts the neutral either in front of or behind the live bar. Some makes have heavy captive tails.
Unless approved by the CU makers please resist the temptation to take a hacksaw to the live bar to make the RCD fit - although this can be made to work it is not recommended as carelessly performed you can remove the integrity of the CU in terms of accidentally exposing live parts, or weakening the case resistance to impact or any number of other potentially dangerous oversights, or indeed just @#$%^ up and cut too much metal off leaving a contact that overheats.
Remermber that an RCD doesn't generally provide overcurrent protection, so the live needs to go via an MCB before reaching the RCD input side- the company fuse alone is not really enough.
Hope this all makes sense -come back if not, before burning your fingers.
