It's quite late in the day, but you may find the attached of some use. It is a simple model that allows you to play with some of the variables influencing the cost impact of PD changes. Note that it only looks at financial impact, and is limited to changes to the number of householders, certificates and enforcement investigations. You could argue that it ignores the cost of change itself, as well as increasing Article 4 activity. If you were in the benchmark, you can drop your own counts and costs (using P0160 - P0180) into the model along with your crystal ball estimates of the other changes.
