Hi, just bought a Volvo for the second time in my life....nice to be back.
It is a 2004 163 v70 d5.The seller told me of an abs problem that started after he changed both rear tyres.The light comes on after a few minutes and stays on all the time.I have ordered two new front tyres hoping that the difference in the front to back diameters is the fault. Am i on the right track?, if it is not fixed with the tyres where do I look next?

 

While this may have happened after a change of the rear tires, if they are original equipment size, the tire change isn't the reason for the ABS light being on. The probable reason is that one sensor (of the 4) is not functioning properly. Replacing the two front tires will give you another 2 new tires, but won't solve that problem. Have the car hooked up to the computerized sensor checker device, get the code, and see if it says "bad ABS sensor".