Environmental benefits

Can you clarify how the environmental benefits of a system are calculated in the UK?

I have seen the other post which explains the source of the data for emissions per kWh of electricity but my questions are:

1. Which set of data is used in the calculations for the UK - Ecometrica or Carbonfootprint.com?
2. Has the figure been updated for the latest data from either source, ie. what year's data is used in the calculation?
3. Can you give an example of how the final figure for avoided C02 is calculated for a project, ie. is this based on kWh generated or the specific self-consumption %age of the project?

It is a question that is being asked by customers and I want to be sure that my response is accurate.