Hi Maesie
Glad to see that you went to the Vet, probably is a diet probably but also may be a problem due to wet weather. I don't know whether you have concrete on your floors of the aviary but due to all the wet weather you are having down there it is a possibilty that your birds have Coccidiosis. Talk to your Vet about Coccidiosis as you could lose a lot of birds with this.
Keep them all on budgie seed, they can can have 10% hulled oats and 10% sunflower in winter and cut it down to about 2% in summer.
Hope that all goes well.
Regards
BB1