ok, started with the simple things first. the bike has been well looked after by previous owner. only done 22000 but i dont know how old the chain and sprox are. i took off the sprocket cover and all is well in there. i tightened the chain more than i should (hardly any play) and eventually the thrashing noise stopped. so i'm assuming its a chain problem to start with. where's the cheapest DID chain and sprox supplier? i'll change them and report back.