It was exactly the same in the local elections in Lambeth last year. In the end Labour swept all before it but the Greens were always considered the threat - and they were; in a number of wards Labour returned all three councillors but the Greens were the next three:
http://moderngov.lambeth.gov.uk/mgElectionElectionAreaResults.aspx?Page=all&EID=25
http://moderngov.lambeth.gov.uk/mgElectionElectionAreaResults.aspx?Page=all&EID=25