Mothercare Nixi pushchair

wobblybox_2

I purchased a pushchair from Mothercare in Nov 08 in preparation for my son's arrival on 19.02.09. For the first 6/7 months we used the infant carrier of the pushchair. My son is now 19 months old and has a job to squeeze into the pushchair. The brand is Nixi and has two solid side retainers that are causing a problem. The Mothercare shop told me that there was a manufacturing fault which is why Mothercare don't sell them any more. They also told me to complain to the manufacturers, which I didn't.
I wrote to Mothercare complaining and they stated that firstly the store guarantee is only 6 months for 'faulty' goods. I didn't claim faulty, I claimed unfit for purpose. This period, however, had almost ran out before my son was born!!! They further state that I had only another 6 months (making twelve months in total) from the date of purchase to complain about the pushchair useability. Not being able to guage the size of my 'future' my son I had no idea what so ever that when he became 19 months old he can only fit into the chair in a T shirt. He looks like a squashed suasage when he has a coat on!!
I'm writing to them again about goods not being fit for purpose as their user guide clearly states that the chair should be suitable for persons up to the age of 4 years (23Kg). My son currently weights 10.4 Kg and he doesn't fit. In my original letter I did state that I had up to six years to complain but they dispute this.
Anyone else have this problem, any help greatfully received. Thanks.