Thanks all... I knew it didn't "expire" as such - was looking for a specific reference to indicate such before the pedants started to shout at me.
I suppose that having gained the experience then you have met the requirements as such - even though the relevance of such 'experience' may no longer be as it intended.
As a point of interest - what is the longest time between "QXC" and issue that you are all aware of?
DD