Well I had no response to my question, but I managed to solve it by deleting the replace history info in my project's **workspace **file. It is also possible to delete the completions history and recent file list from within this file.
Deleting the completions history should re-instate it to its original behaviour - but make sure that you have closed the Project before amending the workspace file!
I hope this information may be of use to others. Andy.