There have been at least 5 parts of the river used for slalom over the years, the ones for the premier division as stated earlier Bala Mill, Graveyard, Top Site ( World Championship) and Ski Jump and they have had lower division event at the campsite below the worlds course.
As far as I'm aware they don't normally use the full world championship course any more as slalom is now over a shorter length ( and a damn sight easier than back in my day 70s & 80s ) I suspect this weekend event will start just above the ski jump and end halfway down the top site course ( at the sharp left corner)
To the non paddler it's all irrelevant as apart from Bala Mill they are all more or less the same place, using the same car parking.
As said above and in my pm to Dave, it's better to get down to water level than shooting off either of the bridges unless using something like a 600mm and aiming way upstream.
Get there in time to have a good walk about and check out the best vantage point ( it'll be obvious as that's where the other togs will be )
I found standing below Mrs Davis's bridge and shooting up stream a good place the last time I was there but it all depends where the gates are set.
Not been for a couple of years, but these are from 2009 and may give an idea of what's available, but I know there are much better shots to be taken there and with the lens range Dave has I sure he get some crackers.