[mumof4]

I want to create some good raised beds for inside my polytunnel. I use any old wood outside but in the poly i'd rather make more effort.

Jewsons are going to cut all the timber pieces for me so i can just nail it all together and they are goining to pressure treat it for me, I'm assuming this is ok.

The timber will be 25cm high - does sound about right.

[Trillium]

Not sure what chemicals they use in the UK to pressure treat wood, but over here no one uses pressure treated wood as food raising beds. The chemicals will leach out into the soil over the years, and spread as you turn over or rotovate the beds.
Ask them for a longer lasting wood like oak or cedar, or better still, use the UNtreated wood lined with h.d. plastic to keep the destructive moisture off the wood. If the plastic dies over the years, you're still safe. You could use the PT wood with a liner, but never without.