Battersea (London, not far from Westminster) - MP (1997-2010) Martin Linton restanding
Crawley (close to Gatwick Airport) - MP (1997-2010) Laura Moffatt restanding
Croydon Central (Outer London, to the South of the centre) - no incumbent (old Croydon Central was Tory held - but boundary changes make this a nominal Labour seat)
Gillingham & Rainham (Kent commuter towns near the River Medway) - MP (1997-2010) Paul Clark restanding
Harlow (Essex) - MP (1997-2010) Bill Rammell restanding
Hove (next to Brighton on the south coast) - MP (2005-2010) Celia Barlow restanding
Portsmouth North (Naval town) - MP (2005-2010) Sarah McCarthy-Fry restanding
A swing of just 0.5% from Labour to the Conservatives would lead to a change of party in all of these constituencies. Of course in real-life swing is not uniform.
The train map for the are can be accessed here.