We assume that you have already powered off the AirPort Express, your iMac and iPad and restarted them a few times to see if the issue continues.
Apple tell me I can't revert the firmware back to 7.6.1.
Apple is not providing you with correct information. If you want to roll back the firmware on the AirPort Express using the iMac.......
Open AirPort Utility (Finder > Applications > Utilities > AirPort Utility)
Click on the picture of the AirPort Express, and a smaller window will appear
Hold down the option key on your Mac while you click on the actual version number of firmware that is installed on the Express
A drop down list of available firmware for the Express will appear
Choose the firmware version that you want to use