My little boy is a bright little chap, he's at full-time nursery and until recently has been fine. However of late he has become detached from the group, aggressive (to the point of hitting members of staff and using strong words such as 'hate' against them) and craving one-to-one adult attention. We are desperately trying to find the trigger to this behavior and so far we can deduce that he 'flips' if asked to do something he simply doesn't want to do and none of the usual methods of ignoring, holding, removing him, seems to have any effect at all. Can anyone out there suggest anything that might help us to work out what it is that is making him like this?

My opinion? Take him out of school. At his age many children still want to be with mom and with adults.
I read an article somewhere (I wish I remembered where, possible Life Learning magazine) that explains that peer experience for young children is actually bad for their social development. They need adults. Trust your child.
There is a chapter that says something similar about what is socially best for young ones in the book Raising Our Children, Raising Ourselves. So, reconsider and I would suggest trusting him and responding to his needs on his terms. He knows best what he needs.