firstly i had a new chain and sprocket and everything was fine but then the rear jockey wheel and derailleur started causing the chain to jump off the sprocket and not hold on any gear could the chain have stretched or do i need to replace the rear gear derailleur i have a shimano sora rear mech which cost only 10-15 quid about 4 years ago any advise on the matter . i was thinking about asking my bike shop to replace the rear mech and cable and also check the chain hasnt stretched , Woold this cure the problem or are there any better options to keep the original rear mech