Chris,
your approach sounds good, but at least it failed practically in Germany. Surely the early Green party in Germany could be called 'marxist-influenced'. Today all left-wingers have left the party, only the careerist around Trittin remained.
Whreas in France the LCR-LO alliance scored some success in at the polls. As I understand it from the distance the LCR-LO success forced the SWP into acceptance of the LSA.
But at the end of the day success at the polls is not an end in itself, but more or less a measure for the maturity of the working class. So I think the 3 percent or so are just the optimal result a left-wing alternative could have got in Britain today. Whereas the success of the LCR-LO alliance is rooted in the French general strike.
Johannes