Thanks - got there in the end.
Dan's method seems to refer to v8 & 9 - my Catalogue tab doesn't seem to metion coordinate systems.
The "Copy and Modify" method was not sufficiently intuitive for me so, for anyone else with this question, here's what I did:
1. Make a new copy of WGS84.prj in windows explorer
2. Open it in a text editor and change the prime meridian to 180.
3. In the Coordinate system tab of the Dataframe properties click on little globe symbol and Import...
4. Select Transformations, then New... and change the "X Axis Translation (meters)" to 180.
note: this give the desired 180 degree translation even though the dialog suggests that the translation is in metres.
and I don't understand why you have to change the prime meridian and set a translation...