Bit of a random one! Does anyone know a good way to track someone running a marathon?

Like a cheap GPS tracker, that you could follow via internet equipped phones for example? Seen a couple on Ebay that might work - £113 the cheapest

I spent all day today, doing just shy of 30 miles on my bike chasing 2 friends around the Bath 1/2 marathon. Finding them was a mare even in their semi distinctive kit

Today I saw people dressed as gorillas, one pair of lads dressed as boobs , Marines in full kit with bergens, and one guy in a Mankini Oddly enough the girls aren't interested in doing any of that.

So what else can you suggest? London Marathon is only 5/6 weeks away, and I don't fancy clocking up 60 miles there following them around

Si