I think it is called scalloping or castellated wear. Basically alternate tread blocks are worn down.
This happened on all 4 Colway mud terrains on my 110 and has now happened on my 300tdi discovery. On the back tyres which are Avon Rangemasters - the original tyres - they had done 48,000 miles no probs - 4,000 miles of me driving and this happens.
Is it me? Tyre pressures etc fine - maybe it is the way I drive round corners.
ANY ideas???