iMac 2017 - Apple Wireless Keyboard 2 and Magic Mouse 2 Bluetooth random disconnects
After spending numerous hours on the phone to Apple technical support, and tried just about every fix suggested here and on othe websites... 

In the process I uploaded some system logfiles to Apple support and after a few more phone calls a Senior Apple technician confirmed that there is a known bug in Apple's Bluetooth drivers which should be fixed in the next patch for MacOS Mojave (which I believe will be 10.14.3). Hopefully this will finally resove the issue.