Oh yeah, and some parties are quite incestuous. People get annoyed with their party and go join a different party then end up returning to tyheir old party. On the left there is a lot of cross-fertilization as well as infighting between the different parties (Socialist Workers, Respect, Communist Party, Real Labour, New Labour etc etc).

When I go to the Trades Club, or go on an anti-nazi leaflet drop, I will usually encounter other labour party comrades (some to the left of me, some to the right of me) as well as a stray member of the SWP/Respect alliance, a couple of ex-party members, the odd Communist and some Lib Dems, there'll usually be a few Trades Unionist non-party members too. At a local level the active left tend to know each other and engage across parties on some stuff whilst tearing each others' eyes out on other stuff :P