How have things been going? Have you hatched any more ducklings or even gotten more fertile eggs? Even if you can't fathom physically going to a university, figure they have e-mail. Maybe try that?
What are they charging £200 to do? The bodies are gone. My idea was just email or physically go there and talk to somebody in their vet department, tell him/her the story and get an unofficial opinion.
My thing is that you don't know what it was. But whatever happened killed every last one...
A quick Google gave me this: https://www.studying-in-uk.org/best-veterinary-schools-in-the-uk/
I am American, but it's difficult to believe that vets, especially farm animal vets, wouldn't necropsy animals that died of non-obvious causes - especially in (relatively) large quantities. You're...
Do you have any colleges/universities in your area or close by - especially ones with some kind of veterinary program? If so, you might donate the bodies there to be necropsied (autopsy but for animals) with the provision that you get copied on the results. It could be that it was just nasty...