Latitude E7250 thinks it's a 7250 (no E)

This topic was discussed a few months ago but the solution hasn't worked for me. I have the problem others have had where Dell Support Assist has installed the wrong BIOS file for my E7250. Dell offers two BIOS files for my Tag number one for E7250 and one for 7250 (why when it knows my laptop is an E7250?). As others have found, this disables a lot of the hardware including two USB sockets, Wi-Fi adapter, power button light and the display once Windows 10 takes over. Fortunately an external monitor always works and so does the laptop display during pre-windows start-up. I have tried everything including a bootable USB drive with the /f /r /forceit parameters. It says collecting data ready for the re-flash and then tells me it can't install an 'E' BIOS on a non-E laptop. I have previously downgraded the non-E 7250 installed BIOS to A03 and tried to install an E7250A03 BIOS as others have said earlier BIOS versions were more tolerant of rejection. I'm reluctant to bin the laptop and not too happy that it was the fault of Dell's own application i.e. Support(non)Assist. This application obviously knows how to install the wrong BIOS so can anyone tell me how it overrides BIOS safety mechanisms BIOSGuard etc?