I'm not familiar with how raw audio data is encoded is .tap and .tzx files – and I don't know how you're creating them to produce this result. I suspect it's probably just a .wav file in a .tap/.tzx wrapper, but I'd have to look into the file format specs to be sure. Anyway, there are various tools that do convert .wav files into 'proper' .tap and .tzx files. I haven't a lot of experience of creating such files personally, but I've managed it successfully from time to time. audio2tape (one of the Fuse utilities) is one such tool, and there are certainly others. If you can't manage it successfully yourself, if you post a straight .wav version of your tape file(s) here, I'm sure that someone would manage to do it without much trouble. I'd be happy to try myself, or someone else might beat me to it!
Thanks a lot. I'm pleased to say that I've been able to convert it to a plain .tap file successfully. Where should I send it to get it to you? NB If you'd like to email me directly, my email address is my name (as shown here) with the @ instead of the space (surname is domain name) and .net on the end.