YES, U are right Joebloggs but i'm only answering adam's specific case that the father was no longer in contact of the kid...as long as ECO satistisfied of the explaination i'm sure it will b ok..
and your right, most of the time there are no problems with bringing step kids to the uk, the only time its not straight forward is when there is still contact with the other parent, but a letter from them saying they wish their child to live in the uk helps alot .