I HAVE JUST BOUGHT HOME A NEW DOG, I HAVE ALREADY GOT A BITCH WHO IS DUE FOR HER NEXT SEASON IN AUGUST BUT I PLAN TO HAVE HER SPAYED BEFORE THEN. TO MY DISBELIEVE THEY HAVE MATED, I HAVE ALWAYS THOUGHT THAT A DOG WILL ONLY MATE A BITCH IF SHE IS IN SEASON.

Are you absolutely sure she isn't in season? 

I would take her to the vet for a premate test.  If she has ovualted recently then the test willshow this and you will be able to ahve the morning after jab, or have her spayed right away, this would be the right time anyway if she isn't in fact in season. 

It is not unheard of but very unusual for a bitch to allow a dog to mate her when not in season and for the dog to be that inclined to do so.

THANKS I SHALL DO THAT. A FRIEND HAS TOLD ME THAT BY FETCHING A DOG INTO THE HOUSE COULD HAVE BROUGHT HER INTO AN EARLY SEASON.