I'm trying to install the Surface Pro 3 TPM update, but I get 'Invalid signature' when I boot from the USB drive

I've tried to follow the (rather long and not straightforward) instructions to update the TPM on my Surface Pro 3, but when I get to the re-boot from the USB drive stage I get the black screen 'invalid signature' error.

I also noted that Step5 of this section says to press and hold Power+Volume Up to get into the UEFI environment. I found this did nothing, and only got it to turn on when I pressed Power + Volume Down (as per later instructions). I presume this is a typo in the instructions, rather than the source of my issues?

Can anyone help?

Thanks, Amy