After an inspired moment and digging around in various mod related files I finally managed to pin down the causes of the Iron Shovel bug!
Fixing it completely involved several configuration changes to two of the mods included with Tekkit, which have now been implemented and tested. The changes do not noticeably affect the functionality of any other items and should not create any additional problems. The nature of the fix means that this problem should not re-occur at all in future.
Enjoy!
