We initially believed we had a 4.1 version to which we would then add SP2.
However RIM bundle the service packs into the latest version to form 4.1.1 then 4.1.2. Our 4.1 being a recent download was therefore actually 4.1.2.
So effectively we were trying to install SP2 twice.
I'm embarassed to have made such a call, but at least it may act as a gotcha for future users!