In case anyone else has this issue, I discovered that you can install using the Microsoft OEM method I mentioned above: on a separate computer, go to dell.com/support and enter the ervice tag on the computer giving trouble, then navigate to drivers and download the IRST package. Then expand this using 7-zip and copy the resulting folder to the USB boot device and start again.
When you get to the stage where Windows Setup asks for the target disk to install, there is an option at the bottom of the screen to load a driver. Click that, then navigate to and select the sub-folder 15063\Drivers\Drivers\VMD. Setup recognises this (I think what it finds is called "RST VMD Controller Miniport + RST Filter Driver") and goes ahead with the installation.
Once you have completed the Microsoft part of the installation, open a browser and go to dell.com/support again and allow it to d/l and install SupportAssist to automatically find and install all the relevant drivers. The only downside I can see (apart from the extra couple of steps involved) is that once it's all done, SupportAssist couldn't install the OSRecovery option they introduced recently. Hardly the end of the world.