I would personally go for the biggest, baddest chain I could find, locked to an immovable object like a lamp post or ground anchor .... Unfortunately, if thieves want it they'll get it! All you can do is put obstacles in their way to make it less attractive. When I go down to visit my brother in Middlesex, my bike has to stay outside, I lock it, using an Almax chain (which imo is brilliant), locked to a ground anchor, I also use an ABUS disc lock, its the alarmed disc lock, which is great so long as it isn't windy, windy weather seems to cause false alarms. I cover the bike and that's pretty much all I could sensibly do.