On Thu, 17 Jun 2010 09:23:22 -0700 (PDT), Andy Dingley
wrote:

Buy better timber, with better rot resistance. Larch or Douglas fir.

Thanks everyone for your replies.

I was interested to read that larch and Douglas fir are more resistant
to rot. Where can I learn more about what wood has what qualities?

What is the best way to lock it shut? Would you use a padlocked bolt
or could you somehow incorporate a mortice lock?

Padlocked bolt. Too skinny for a mortice lock, rimlocks are generally
flimsy. If you have a substantial and weatherproof rimlock (some Yale
pin tumbler deadlocks) then use that.

Rim lock is what I meant. I knew mortice lock was the wrong word
because I knew the mortice was the slot it fitted into but I just
didn't know what the right word was. Thanks. A couple of posters have
suggested padlocks being best, so I will go down that route. The only
disadvantage of a padlock is that it only really allows unlocking from
one side, unless you cut a hole in the gate.