Thanks for your help Bob, but I nee a little more. To test the measurements I strapped the PT100 (ceramic cased) together with a thermocouple temperature monitor to the barrel of a temperature-controlled soldering iron and covered it with a thermal insulating layer (a bit of a lash-up, but it appears to work). However, it would appear that I have a significant gain error and a significant offset, even allowing for potentially poor heat transfer from iron to PT100 and thermocouple.
I am about to check EVERYTHING - component values, register settings and mathematics. However, I am not sure about the IDACs. They are both set to 250uA, but when calculating resistance (Output*LSB/I_IDAC)+100 do I use 250uA or 500uA? I am using the standard quadratic formula to solve the Callendar-Van Dusen equation and that checks out, so I am assuming that either the calculated resistance value is wrong or the data returned by the ADS is wrong (which I have all but eliminated.
I am quite happy to correct for Offset and gain in software, but as this is not my area of expertise what typical values could one expect to correct for the sensor?