Here are the results in Brighton and Hove for the 2005 General Election and the following local election in 2007 (I have used the top candidate from each party as a rough guide to their number of voters in the local elections).
If we extrapolate these results onto the 2010 general election results we can make a prediction for this May as follows. I have added in a further adjustment for the current polling in the third bar chart. This suggests that the Tory vote may well hold up, the Labour vote will recover a little and the Greens will do very well indeed. The Greens being the only party to attract more voters in the locals than they do in general elections (mainly because their vote is squeezed in Hove and Kemptown due to Green voters switching to Labour in an attempt to stop the Tories...alas in 2010 this was invain.
Surprisingly these vote changes may make little difference to my predictions in terms of seats (once again welcome to our distorting first-past-the-post system where the biggest minority party can take all the seats with a 30 something vote share) - I still see the Greens gaining no more than 6 seats, the Tories losing 1-3 seats and Labour losing 1-3 with the Lib Dems wiped out. We shall see. CON-25(26), GRN-19(13), LAB-10(13), LD-0(2). Vote Green in Preston Park, Goldsmid and Brunswick+Adelaide. Vote Labour in N.Portslade, S.Portslade and Moulsecoomb+Bevendean to stop the Tories and I expect all the other 15 wards to be safish, with the Greens consolidating their lead in Queens Park and Labour holding on in Hollingdean+Stanmer from the Green challenge, being the only other two wards worth a look.