In March 1991, Anthony McGeehan wrote an article in Irish Birding News ('Brunnich's Guillemot: rare or overlooked?'). In it he wrote of the different pattern on the underwing coverts between Common and Brunnich's: '(Common) Guillemot has the most variable underwings. The great majority look both 'dirtier' than either Razorbill or Brunnichs, whilst also differing in the detail of their pattern. The dark underwing markings are most pronounced on the inner part of the wing, due to the axillaries being dark, and there is normally an obvious smudgy dark 'spur' (formed by the bases to the median underwing coverts) projecting into the white of the underwing.'
Hugh Harrop has just posted some great images from a recent trip on his website, which illustrate this nicely and so I thought others might like to see them.
Common - http://1.bp.blogspot.com/_90oyWFBpxo...Guillemot4.jpg
Brunnich's - http://2.bp.blogspot.com/_90oyWFBpxo...Brunnichs5.jpg
Common and Brunnich's - http://4.bp.blogspot.com/_90oyWFBpxo...Brunnichs2.jpg
The markings on the flanks can also be seen. A little less obvious is the rather neckless appearance of Brunnichs.