Hi Ian. I read a few articles about this last winter. The consensus and my conclusion is that rotation of tyres is an American fashion where, I believe, tyres often last longer than the UK, partly as a result of a guaranteed minimum mileage. Others may know more about this than I do. Tyre rotation used to be prevalent in the UK until about 25 years ago but now seen as not necessary. However, the consensus was that new tyres should be replaced preferably in pairs on the same axle (as you do already) and placed on the main driving axel. I consider this to be the rear axle even for 4WD cars. The partly worn tyres should be moved to the secondary axle (front).