I think I must have messed up the permissions of the /usr/bin/sudo

any ideas? I can't do su either, su must be suid to work properly

I don't understand your question fully, I'll try help:

Just type 'root' or you can use sudo when doing something.

'sudo apt-get update' or similar.

Permissions wrong?
chroot can fix permission's. use google or search here.

stat `which sudo`

Access: (4755/-rwsr-xr-x) Uid: (0/root) Gid: (0/root)

It kind of needs to be suid root,
su won't work because it is a link to busybox.. which is in itself not setuid root

That is a bit disappointing.. if you've broken sudo, lots of things will stop working.. have a look at /etc/sudoers for some info.. ok.. that's not readable by anyone other than root.

If you can install packages, I'd install the openssh-server, then log into your phone from a pc.. but I have a feeling that installing packages requires a working sudo..