Thanks for the replies. I have looked at Tanalith E (this is Chrome and Arsenic free) and is claimed to be safe to use which I don't doubt. I am approaching this more from a point of 'is this treatment actually necessary' if there is a viable untreated alternative for this application. The HDRA webs...

I would like to put some raised beds on my allot ment and would prefer not to use treated timber. Larch is often recommended as having reasonable longevity. Does anyone have any experience of this or other suitable wood types? Thanks