A further thought to the above for Simon, is as you don't necessarily want to progress to big stuff as I do (which needs the stability the double-bar-clamps either end of the chainbar that the Alaskan offers), you be might be OK looking at the Logosol Timberjig for £154.
http://www.logosol.co.uk/sawmills/big-mill-system/big-mill-timberjig.htmlThe benefit of it is its cheaper than an Alaskan, you can use the full 20" length of the bar too. The downside is having seen the timber it produces edge-on, its not as rigid as an Alaskan, the chainsaw bar can wander more easily as its only guided on one end and the other is free, and the result is I noticed a bit of a taper on the cross sections of the timber- looking at them end-on, some were slightly wedge-shaped. If you wanted to produce something precise and start with flatter planks, the Alaskan wins. (or you might want to put the planks through a thicknesser/planer to take off any unevenness.)
If doing squared off timber such as posts or straight edged planks, you need either an Alaskan Mini Mill which does vertical cuts and can take off the sides from a log, or to consider the Logosol 'Big Mill Basic', which is a Timberjig but with a metal guiderail held in place by rotating clamps that are screwed into the ends of the log, so that you cut off the top and two sides before planking squared timber. The Timberjig can also do squared off edges, but it needs a bit of faffing with homemade wooden guide-rails, unscrewing and rotating them. Theres also obviously the option of starting with waney edges and then later on using a circular saw to take off the edge.
I trust with wanting to do compost bins you're not too fussed about seasoning- just make sure you take off the sapwood. The best advice I've seen on seasoning is Rob's 'wood drying' section on the Alaskan Mill site. It needs a bit of care & the right space and kit, but its not the mystical dark art that some make out, with a bit of common sense and patience, its doable.