Hi Ben,
your post is a bit older now and I am not sure how you have fared since then. Tim kindly linked your thread to me recently as I am just going through the EPC ratings while I am doing a partial retrofit.
Here is the thread and my latest post about the EPC ratings. As they have been just updating RdSAP to 10.2, ASHP may be better served. Retrofit project planning starting with underfloor insulation - #81 by Christian_Unger
However, there are may quirks and you likely need to be an EPC assessor for your own house to get the values entered correctly or you better prepare yourself and the evidence for the EPC assessor so this can be added correctly in the short time they have available.
I am planning to do another assessment when we are done with the current phase 1 retrofit, and I was hoping I could get to a B rating with all the money spent. However, I am not sure if it can be done.
Let us know if you have dared to do another EPC or are still with your older C rating. Thanks.