I have just bought a 2nd hand M4 Rockhopper which has a dent in the seatstay, it is right accross it and I guess about 3mm deep. Would you risk riding it, I'm about 12 stone???

yes.

Is there any creasing?

I'd say it is a bit creased,

I wouldn't have bought it, which is possibly the more relevant question here!

Yup – unless you knew about it or it was very cheap I'd be after my money back

Even if it does fail it is unlikely to be catastrophic as there are two seatstays

Where abouts in the seat stay as well? Mine has a funky bit of shaping on the drive side that does look a bit strange until you get used to it…

I got a decent amount knocked off it when I noticed the dent, it is just whether to ride it as is or swap the stuff over.

I reckon I'll ride it, just not too far from the car though 😉

Wouldn't have happened with carbon