Yeh i will be monitoring things very very carefully.  The dogs have almost tripled in size over the past 3 weeks/month.

It was exactly the same behaviour I have seen when taking dogs pig hunting, once they latch on, they just wont let go and shake their heads side to side violently. I was completely shocked this afternoon because there were a few adults in the backyard and the dogs still had a big go with no notice or reason whatsoever...

Might contact a staffy dog website/forum and get some advice from a few experts. I am sure some will say that they can be trained properly, and some will say the opposite. Time to make some serious enquiries.