Nope - that's the point, there isn't a blockage! The drum drops straight into the pump, and the pump simply squirts up the outflow hose. The drum empties quickly if I lie the outflow hose on the floor...
The pump runs briefly until the controller thinks there's a problem... I'm inclined to think the controller is at fault, which is why I'm asking if anyone else has seen this problem.
The only 'valve' is a flap of rubber on the outflow of the pump (similar to the non-return valve on bagpipes!), but that is behaving normally.
I was expecting the usual problem of a hair clip or bobble in the pump (my daughter's is always suffering that way!), but it's as clean as a whistle.