Come to this board a bit late, but felt the urge to post..
Jbr Hope you get this message deletion problem sorted out. For your information, when I'm not at my trusty computer and have to use an imposter, I, too, can find it difficult to post messages. Additionally, I don't think that my messages via 'contact us' or the notice board have ever got through - a dodgy link somewhere? I don't know. If you find out the cause, please tell me.
Toasty, in reply to your message I just have to say that my first reaction was one of anger that your son's nursery: 1. Told you about his behavior, and, 2.Threatened to remove him from nursery, without warning, BY LETTER. Surely they should talk to you about this, even by phone? If this is how they tackle things, no wonder they have 'problems' with their children. End of rant.
Anyway being more postive, I think Bloom has given you some excellent advice.
As a fellow SE Londoner with 2 sons who have gone through a variety of local childminders, state and college nurseries and primary schools, I can say this: the style of care, if not the quality, does vary a lot. There are so many factors that are going to influence your son's behaviour, from the amount of runaround space to the personalities of his classmates. I would never go as far as to heap blame on your nursery for your child's behaviour, but IMO there's every reason to see it as a passing phase.
Do get any help you feel you need, and, most importanty keep the lines of communication open between you and the nursery. Ask them before they tell you.
I see your son is four, so he'll starting school soon. One word of advice - if your nursery suggests a suitable school for your son, don't let this determine your final decision. My oldest son left his local primary at 5 years to go to one nearer our new home. I was guided by his head teacher's recommendtions, despite visiting a variety of schools. In hindsight I should have trusted my own judgement a little more.
Good luck.