Hi Rich,
I just had my gearbox oil changed at 68K, it is recommended to do this at the 65-100K mark, mine didn't really need it either, it was more of a preventative measure.
I actually called and spoke with a ZF engineer as some people were suggesting it was best to flush and replace the oil but the ZF engineer said they don't advise to flush as this can loosen some debris and cause problems, it's best just to drain and replace the oil but he did say it is important to change the oil and not sealed for life.
I had mine done by E&E in Milton Keynes, if I remember correctly it was about £450.
Hope this helps,
Dom