Glenn,
Simon Lester at Woodford garage has known this happen before and recommends the ISCV, if removed and cleaned, that a small amount of thread lock is used.
So take it back out and make sure this does not happen to you.

PS; Ive owned the 500 for 3 years and have never removed the ISCV, so I think im just unlucky..........oh well its only money.

Steve