Broken valve spring

Broke a valve spring on my 300 tdi discovery. Replaced broken valve spring and collet. All good and less than 70 miles later the same valve spring has broken again. What could the possible cause of it happening again on same cylinder.
Could it be a bent valve that's caused. It to break again as can't think of anything else it could be.
Any advice would be grateful
Thanks guys

anything else done recently

Replaced head due to head gasket going before Xmas which had cracked head hence replacing the head