When mine went it started making an intermittent chattering sound as idle, the noise would go away completely when the clutch was depressed, easily mistaken for a duff slave cylinder bearing. When i removed the DMF it was well gone, you could easily press on the edge of the front face and it would make contact with the rear plate, this is what makes the noise, the two plates touching eachother. I sourced my fly and clutch etc from CFM in america but since then i am told the TTV now produce a solid fly wheel for the ST170 that accepts the larger clutch friction plate, make sure you get the fly wheel for the 240mm clutch plate and not the smaller one otherwise it will slip. Early ST170's had smaller clutches 220mm and they began to slip so the later models got a larger clutch plate.
If you go for a solid lighter fly like mine be prepared for clutch chatter, the lightweight fly allows the gears in the gearbox to rock and chatter against eachother at idle, the noise goes away when in gear and with the clutch down but it is quite loud, it sounds like your gearbox is well broken. I will be honest it gets on my tits quite a bit but it transformed the car