I have read the posts previously that state there is a safety mechanism where the spring will collapse and turn cherry red if there is a battery failure. I guess this is what happened with me - the battery all of a sudden started rolling around in the unit as if it was too small.
However the spring was not cherry red, and I was confident the battery is still alive, It does appear that the spring collapsed though, So I just took my screwdriver and stretched the spring back out and it works fine again.
Though I am now worried that this spring will not collapse again in future if there is a real failure, and the unit will explode in my face, this is putting me off using the phidias even though it is working fine.