Well the underlying issue of my previous post was the fact that, when creating a calculated custom field referring to the system field "Modified" as part of the calculation in a document library, the value would change to 01/01/1899 when a document was modified.  This was very annoying so I attempted to find workarounds, although to no avail.  I have subsequently discovered that if you create the same field within a custom list then it works fine when a list item is modified.  This is obviously a bug and one which I feel Microsoft need to fix.

Hope this helps anyone who may be having the same problem and it stops you going down many dead ends like me.