I think my trailer bearings are identical to yours, Mike - alko sealed on my Bramber trailer.
Before I picked the boat up from Humber, one of the locals down here suggested I get a spare set of bearings for the long 9hr journey back home..... I was then told that apparently, they are guaranteed to last for 25,000km which will easily last my lifetime of having the rib on trailer.
I do have to wonder though exactly how long the sealed units will last for, cos as you say, they will be an absolute s*d to change at the side of the road
-Alex