Ok all done. Wasn't too difficult. I followed the tip of tieing something to the end of the old cable (I used a cable running tool called a Gardner Bender - from Home Depot, it worked well as it's semi-rigid). I we pulled the cable through from the helm (as you can't get the rack end through the bulkhead behind the helm).
I've attached some pics for anyone who's planning to change their own cable...
Pic 1 shows the rack part of the cable (this bolts to the pinion on the back of the steering wheel, a replacement is part of a new cable).
Pic 2 is the tool I used to follow the old cable out and pull the new one through
Pic 3 shows how the rack is bolted to the pinion at the helm (steering wheel)
Pic 4 shows the helm / pinion with the rack removed (it's just four bolts)
'92 190 Prostar 260hp Indmar.