I've been told by a garage that I need a NSF wishbone due to a knocking noise. The Punto is 13 years old. is it safe to drive until I can get this done?
It may be safe to drive, but it's all dependent upon how worn out it is. Since it is a crucial suspension item, I'd highly suggest you do not risk your safety or the safety of your passengers. Either get it fixed or don't drive it until you do.
On a 13 year old Punto the answer is "No". Have the wishbone replaced before driving the vehicle. You have your own personal version of Russian roulette going on at the moment.
May be you will have a sudden failure.
According to the things you mention, I would listen for noise. If you get a knocking sound when you go over a pot hole, or possibly when you corner hard, that is time to change it.
In the UK it may not be legal to drive knowing your car is technically not roadworthy as parts of its suspension are not sound.
Its also unknown when the wishbone will fail completely or even if it will fail completely at all.
At around £25-£50 for the parts it would be sensible to not risk it and get it replaced - small independent garages may be cheaper than dealers to get it worked on if money is an issue. Additionally it may be putting stress on other parts of the car as its not able to bear its load correctly.