As far as I've been able to identify, they are:
Customer #1:
Ruth Sheen as Mrs Webster;
Customer #2:
Rick Warden as Gordon, the guy who's looking for a very special plant and ends up with a cactus;
Customer #3:
Justine Mitchell as Laura, the emotional woman who's planning to tell her boyfriend that she's pregnant;
Customer #4:
Andrew Dunford as Mr Cornell, who wants to start a herb garden;
Customer #5:
Ben Willbond as David Steele, who is looking for a break-up plant;
Customer #6:
Mona Hammond as Mrs Edwards, who is told by Luce's mother to keep an eye on the store.