Now you're gonna blind me with science Peter - lol
I've just spoken to Cookson and they asked me to send back the sheet to be tested. The guy I spoke to did say he thought it was unlikely that I had over annealed it but will wait and see what the tests reveal.
Just thinking aloud - surely when doing several solder joints on one piece, its like annealing several times without actually working in between? so in theory, this could happen to something like say a ring that has had several joints soldered, and maybe a bezel cup soldered onto it. During each soldering process the metal is getting as hot as when I am annealing?