The 'Punkbirder' boys have put together a superb photo-essay illustrating the identification and covert-bar variation of BLACK-BELLIED STORM PETREL at the following link
http://punkbirder.webs.com/severnpetrel.htm
This is the UK400 Club Rare Bird Alert highlighting all records of avian interest and published in association with Rare Bird Alert Pagers and utilising additional information gleaned from the Regional Birdlines, BirdGuides, local email groups and individual observers