As title really, I have a suspicion (mainly because the car has over 130k miles on the clock) that the suspension may be tired, but if it is, how would I be able to tell?

how does the car handle? have u inspected the shocks for any leaking? is your car on standard susp?

when i bought mine it was quite soft and wallowed a little, i changed it to a full H&R kit with all top mounts etc changed, made an amazing difference

The car handles pretty well I think, although obviously I don't have a one with brand new suspension on it to compare. However, I would say that the suspension is relatively soft (but firmer than the vRS was), and there is a fair amount of body roll there - my mate who was following me said this was surprisingly noticable.

Is it worth going for an aftermarket kit for the Golf GTI or is it worth just keeping it standard and going for new shocks and springs?

depends what ur after, if u want even better handling and the car to sit a little lower i would go for it.

stick to either the H&R kit or eibach springs and Koni shocks on the mk2, it makes some difference