Ian Gledhill released a new program to transfer recorded cassette files (WAV files) from various computers into the appropriate image file, which you then can use in an emulator. Currently it supports the C64, VIC-20, C16/+4, Acorn Electron, BBC, Sinclair Spectrum and the Amstrad CPC of course. The utility also comes with POSIX-compliant source code and binaries for Windows, Linux x86, AmigaOS 3, AmigaOS 4, AROS PPC, AROS x86 and MorphOS. Currently the utility is a command line only program, but the author may add a GUI in the future. Give it a try and try to convert your tapes with this program and don’t forget to write about your experience with the program in the CPCWiki forum.
You can find examples and the utlity on the homepage of the author.