Hi whitewing, sorry your question dropped down the list and I'll try my best to answer although I'm not an expert on birds;
from what I understand once the bluetits fledge they will be fed and remain close to their parents for the next few weeks as they are very vulnerable soon after leaving the nest-box at that young age until they eventually become independent. In general blue tits never move more than 2 or 3 miles from their place of birth so most likely remain in the general area, maybe even the same garden if it is large enough to provide several territories. As for finding a breeding partner, all down to nature but even whilst the female is building her nest the male partner keeps within close range of his "lady" in case another male fancies his chances with the female ! As far as I know the Blue Tit fledgling will be able to breed in the following year.