Just eaten this and the shan mix is very nice the chicken was a bit dry but tender I was able to cut it with a fork, had it with salad and Dip's mint sauce which is as good as if not better than most T/As, if you haven't tried the sauce give it a go you won't be disappointed just leave the sugar out. Maybe our ovens are at a different temp when set to 180c. I'll be using your method again but maybe drop the temp a bit.
Regards,
London.
PS Shan mix 69p for 50g only used about 3/4 of the pack for 6 large chicken breasts.