Domestic RHI - heat demand calcs EPC vs MCS

We've had a heat pump for a couple of years and we can now apply for the domestic RHI. To do this, we need an updated EPC which shows a calculated figure for heat demand. That is then used to calculate the RHI payments. DECC do not allow the use of the MCS installers, room by room calculations to determine the heat demand.

The issue we now have is that there is a very substantial difference (~30%) between the EPC heat demand figure and that calculated according to the MCS methodology, the latter being the much higher. There have been no changes in our property between the two methods of calculation.

I have been through the MCS calculation and can't see obvious errors. The EPC figure is essentially a SAP calculation which is based more on the overall property size and nature of construction, but generally less detailed.

Having had a look through the 200+ page supporting SAP calc document, the only real issue I can see is that it appears to assume a duty cycle of 16 hrs at weekends and 9 hours during the week (2 hours am, 7 hours pm). In fact, as we in part have u/f and because of the low grade heat, we run our system 24/7, which would easily explain the difference. I would imagine most heat pump users would be the same. Certainly, we can't switch our system on/off like that because the system latency would make the system unusable.

Has anyone come across this before?