nettle fertiliser

I'm not going to say that I'm 100% sure about this but I don't think that the nutrients that you got from the nettles have gone anywhere  - having said that there may have been a chemical change over time that has neutralised them but I'd suspect that they are still available in the tea. Perhaps I should have become a biochemist... too late now. 
You could apply it to some plants and not to others - see if it makes any difference. I'd chuck it on personally.
Pip pip,
Balders