A much, much higher proportion of conductors has gone sick elsewhere, though. The explanation is certainly correct for some routes. The problem is that it (or its indirect effects) has been used as a blanket explanation for services where it is totally irrelevant, too.
I would say, re. Brighton, that is an unusually low number of conductors sick compared with some other large depots, so well done to them. Though it should be said that the levels will fluctuate throughout any given day, as the printed weekly roster only shows those sick at the time it was created, and can't predict the day-to-day circumstances of people calling in (duh).
Brighton has roughly a 1/3 of all Conductors so just shows how flimsy the GTR sickness argument is in blaming them,it has been seen through by a lot of the public too.
When I was rostering train crew it was pretty easy to predict sickness and the day to day circumstances but then again I was allowed to book overtime and rest day working. If I hadn't had these resources even with 4 sick would have made the job very difficult !!!