Managed to get this to work so I thought I'd describe it briefly in case others find themselves in the same position. The difficulty is that Apple deprecated FTP in Lion in favour of the more secure SFTP which is now enabled through System Preferences > Remote Login.
To set up Textastic just create a new SFTP (SSH) Connection. The host should be the IP address described in Remote Login ( eg 192.168.1.9). The port should be 22. The user name should be as per Remote Login (eg johnsmith) and your password should be what you use to make admin alterations on your mac. Everything else I left as default.
Once the connection is made you'll see all the files on your mac so just browse to Applications > Mamp > htdocs to see your server's files. Download what you want to edit and just make sure you upload them once you've finished!
Hope that helps.
Customer support service by UserEcho