Ok, Ok I had it all wrong.
On return to the nest last weekend the situation quickly revealed the family's structure.
On arrival there were two osprey on the nest, crying incessantly. That makes them chicks but I never would have guessed it by size.
For some time they just sat waiting for their fish to be delivered.
At last they got off the nest and flew around chasing eachother in what could have been play or could have been a more sinister kind of sibling rivalry.
It's not uncommon in raptor nests for the stronger nestling to do away with the weaker, but that's much earlier on.
These two are pretty much full-grown and not likely to risk injuries from a battle with a nest mate.
As they flew, the rope-tangled talons came into view, meaning it's one of the young with the problem. But she's huge. I would never have believed that was this year's offspring if the parents didn't come along shortly thereafter.
Until then, the fledglings soared around taking pot shots at eachother but once they heard the call of a fish-bearing parent they rushed back to the nest.
From that point on it was a steady stream of what looked like perch brought over by both adults.
Five fish came in the 30 minutes I stood there watching. The adults were still out fishing when I left to get my dinner.