As far as I can think there has nothing gone wrong with the car.
The only complant I have it the car does take a long time to warm up from cold mornings, but diesels engines do run cold anyway.
Very cheap and comfy car, I have had my car for 8 months now and still don't have any problems with it. Its very quick when you need it to be and very cheap when you take it easy.
60mpg is easly achieved.
I have a jtd punto too and I know what you mean about it taking ages to warm up.
Can you post some information about the turbo on your car as mine does not provide any boost until the engine is at full operating temperature. This means that on short trips the car performs like a non-turbo straight diesel and therefore has very poor acceleration.
Does your punto behave in the same manner as mine?