I eventually found two solutions:
- To connect an external mic to the headphone jack using some kind of adaptor - using the same mechanism as it uses for headphones with built in mic for taking phone calls. These adaptors are available online.
- To get an external camcorder and then copy the files to my phone via CloudFTP. This was the solution I eventually went for. CloudFTP is a great device and has been very useful to me. (For anyone who doesn't know, it creates its own access point and makes available an FTP server on that access point with the files of a USB device. The camcorder I went for - the Zoom Q3HD (which seems to be the best budget camcorder for audio recording) - has a built in USB which allows me to connect it straight to the CloudFTP).