I have two beautiful staffordshire bull terriers male and female Holly and dannyboy we did intend to breed them next year when holly is 2yrs old, she is now 1yr and just in her 2nd season. We managed to seperate them, however one of my children accidently let her out in the garden and in the space of 5 mins her and danny were locked into posistion bottom to bottom she was 12days into her cycle. What is the chances of her catching on pregnant, how soon can i tell and most importantly will she be ok as she is only 1yr old though its fair to say she is a very good sized healthy lean irish tiger brindle, im still a little worried for her what do you suggest? help! 

Personally I'd take her to the vet for the canine equivalent of the 'morning-after' injection.

Yes i would take her to the vet and have the injection done as well.
She is still only a baby herself.
if you are planing on breeding fromher when she is older, you need to make sure she has had all the relevant health tests done.

Also you will ideally need to board your boy away.  After the injection and now that they have mated it will be even harder to keep them apart, and this will be a problem next season and every other season when you don't want them to mate.

Also how do you know that they are a good match for each other?  It is rare that the dog and bitch you own yourself are going to be the best pair to produce the best quality pups.

Apart from Hip scoring eye testing, and DNA testing for another health problem for which there has only recently been developed a test you will need to have them both assessed by a breed expert as being suitable quality to be bred from.

Are you aware that there are a huge number of this breed in rescue?

One poster recently went into her local rescue and out of 42 dog there 37 were Staffies or Staffie crosses.  Do you really have a good reason for wanting to breed?

I suggest you look at this article/chart:
http://www.dog-play.com/breedercomparison.htm