Soak them in Creosote, get a local friendly farmer to get some of the proper stuff from his local merchant.
Won't that make the inside of the Summerhouse a bit smelly for the first few years? I'm definitely in the 'lots of air flow' camp. We built a log cabin back in 2007, none of the timbers were treated in any way (although it was high-quality Scandinavian timber) and it's still completely sound as there's a good 250mm air gap underneath.
