Thank you bremner, most helpful.

Its fine at present but gave me thought abut replacement when like a slap in the freeview box faile to find any signals during this weeks retune.

A good and very savvy friend gave me a simple answer which might just help others with systems that are functioning fine until it is time to re-tune.

Pull the aerial connection out so there's no possibility of a signal reaching the freeview box.

Carry out the re-tune as far as failure to find any signal. Replace the aerial, and run re-tune once more. Hey Presto!