Just a thought, although nugget is neutered and has been a while I believe they can still get 'urges' so perhaps bluebell is giving off the scent and he isnt liking it? Or perhaps she's marking territory and this is causing him to become more aggressive as it's his area?
Bonding should be done in neutral territory and unless I've missed it, I believe you've put them in his cage?
It could be a case of you've moved to fast and left them together too long. Hopefully someone with more knowledge will enlighten the situation