I've setup the source file mapping so that I can debug a problem that appears somewhere deep in the Qt internals - but I'm not seeing the Qt source files associated with the disassembly.

The disassembly window is saying the missing filename is "c:\users\qt\work\qt\qtbase\src\corelib\tools\qbytearray.cpp" 

Which exists on my install in "C:\Qt\5.11.2\Src\qtbase\src\corelib\tools\qbytearray.cpp"

So I have setup the "Source Paths Mapping" to map "c:\users\qt\work\qt\qtbase\src\corelib\tools" to "C:\Qt\5.11.2\Src\qtbase\src\corelib\tools" - but no source file appears

It's not something has simple as the filename matching not accounting the Windows and Unix slashes in the path component?