I have been trying to tighten security on a Speedtouch 780 WL router.
I have enabled WEP,, and it is working ok. I then thought it would
make sense to add an administrator password.

Can it handle WPA instead ?

WEP is trivially easy to crack as it transmits the password in plain
text, so all someone has to do to use your net connection is "sniff"
enough packets to see the password. There are even programs that
automate this detection available for free.

An additional, although it probably still wouldn't help much :-} would
be to lock down which MAC addresses are allowed to use the router -
but again, it's relatively easy to detect the MAC of a machine, and
you can "tweak" the MAC address of another machine to make it think
it's one of the allowed addresses.

As far as the admin password is concerned, as long as you don't have
remote access enabled, you should only be able to access the router
config "locally" - you may be able to tell it to only accept admin
requests from a machine with direct cable connection. If they've got
that far, they're already in your house, and you have a bigger problem
than them trying to **** around with your router settings :-}