Just to say that we have concluded the mdf file is beyond repair. To share the scenario again

With the bad sector in the mdf file

* the T-Log backups succeeded

* the Database was still accessible/functioning

* and CHECKDB were appearing good

However

* Full & Diff backups failed

* the MDF file could not be copied when the DB was detached

* the DB could still be be reattached in situ

Due to some poor file management & the delayed identification of this whole issue

* the log chain become broken (due to limited log backups retention)

* the only solution was to restoring an old backup and painful copy out of the data

Thanks Gail for you responses.