@Jimpson ,
for me it was in ~/.local/share/ubuntu-download-manager/com.mikeasoft.podbird
but if its not there, try using the du command to search for large directories. I started from the very top at ~ and started using this:
du -d 1 --block==1G^this command shows the size of each directory/file in ur current location in terms of gigabytes. podbird should be up there in size, so just start working ur way down