I seem to remember having seen something about great tits with a dark head before, and in the last couple of days one has visited our garden peanut feeder a few times. I'll try to get a picture of it at some point next week, but I'm a bit busy with exams just now. 95% sure it is a great tit, but if anyone knows anything more about it, that would be great.
Fair chance it's an adult that has been feeding young and most of it's head feathers have worn away giving the dark headed effect.
melanistic birds were regularly-ish seen in Surrey ..