Caitrona I had exactly the same problem with mine. Mine were worn out within about 7 months...i know i walk alot but i thought the wear was a bit excessive aswell. I never got new wheels though as i wasn't sure how to go about it so we just got a new buggy.
I got in touch with Graco and they waffled about warranty being 6 months but I could buy new wheels at a cost of £9 each. They did supply a phone no as well.
I am not pleased at the thought of paying another potential £20 on the cost of the travel system. The main web site did not mention spares cost or availabilty.
I will probably get the set as it doesnt seem worth a whole new buggy for less than a year. I hope you enjoyed your second buggy and have fun with Joe and new baby.