I thought it would seem a bit OTT but I've not had much luck in the past when I've tried to use code.A bit extreme, does the services menu show whether betaftpd is running?
Just use Putty, Telnet or the webshell command line (on diagnostics page) to give you a linux command line where you can rename the errant hmt file
You should then be able to use the sidecar tools to rebuild a valid hmt.
Having said that, I got lucky this time and after a bit of googling and reading the wiki, I managed to delete the original hmt file via Telnet and then recreate a new one using sidecar.
I'm happy to report that everything is back to normal again.
Thanks everyone and apologies to Andrea Edwards for not taking your suggestion more seriously.