They do this by asking for details of not only your size, but also your build - and they'll send you a tape measure of you need one. The sizes progress in increments of 1 inch between 30" and 36", so there is generally a size and fit for all.
Mine are navy blue and are a comfortable, slim, but not too slim, cut. The fabric is a soft Italian fabric which comes in a variety of colours. Best of all, they're stitched together in Hoxton, London - so these are genuine British-made trousers - all for £79.
See Spoke's website for more information.
With thanks to Spoke for providing a pair of trousers for use in this review