This was Tina Cole's third appearance on the show, each time playing a different character who was friend of Robbie's. Starting the next season she would become Robbie's final girlfriend and then wife. This one is a kind of foreshadowing that the two would later marry and have their own children.
This story is basically a remake of two earlier ones, where older brother Mike and his girlfriend Sally played the would-be parents and wondered if they could handle being married.
Tina Cole said on Facebook, that this episode was essentially her screen test when it came to casting the character of Katie.
Ernie mentions being new to the family to an obvious adoption reference one of last, in the series