Hi Simon these N series engines have lots of timing chain problems combined with valvetronic problems. When you changed the chain did you also replace all the guides & modified tensioner? were the parts genuine BMW parts? If the chain is genuine did you do a reset on the valevetronic system?
The eccentric shaft & valve rockers wear badly on these engines & are mega money & time to repair requiring specialist tools again. Autologic have a facility to compensate for wear in the eccentric shaft, but once the shaft & associated parts wear beyond their limit the engine can run like a bag of nails, even to the point of non start. Unless you have the software these engines are difficult to do much with.
John Kennard.