It's at Highbury Corner. That's in Islington, where Upper Street and Holloway Road meet
Directly opposite Highbury and Islington Tube (Victoria Line). Get out, walk forward and look for a big red 'Bass' sign on the pub which is across the big square roundabout. Also walkable from Angel (Northan Line) walk 20min up upper street, and Holloway Road (Piccadilly line) about 20min south down Holloway road.
Numbers: 43 (London Bridge -> Highgate) 19 (Battersea Bridge -> Finsbury Park) 4 (Waterloo -> Finsbury Park) 30 (Marble Arch -> Hackney Wick) 277 (Canary Wharf -> Highbury & Islington) 271 (Liverpool Street -> Highbury & Islington)
The pub is quite small and tends to be only used for quick get-togethers rather than full meetings.
The pub often stays open late with very noisy bands kicking off about 10:45pm and playing anything up to 2am.
Flaky wireless (802.11b) may or may not work in the pub. The management don't know this.
The Hen and Chickens has a wonderfully bad web site at www.henandchickens.com