With the Samyang/Rokinon/etc. etc. 14mm f/2.8 lens, a few notes: make sure you get the later UMC version. The original MC version wasn't nearly as good optically. Make sure you have specialized software (like
PTLens) that can correct for
the wave/mustache distortion (it has barrel and pincushion distortion combined. Simple lens distortion correction, like the kind that Photoshop/Lightroom can do, will not be sufficient to fully correct for this lens unless you're using a lens profile in ACR.)
You also need to realize that you cannot use filters on it, it's one big honking piece of glass, and it's slight overkill for a crop camera, as it's designed for full-frame. The cash
might be better saved towards a Tokina 11-16/2.8 instead if you're shooting crop, and non-CPU lenses are too PITA-laden for you. While it's more expensive (£528.98) and it won't autofocus on a D3100, it
will meter accurately, not restrict you to M, goes a lot wider, is just as fast, and can take filters.