It's best to use a NON-ISOFIX group 0 rear facing baby seat to start off with. Babies just grow too quickly. This is what I recommended to Paul

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Then buy a Britax DUO ISOFIX forward facing for £160 (available Baby's R Us). They say that certain seats aren't recommended by BMW and not tested by them. But the majority of car seats haven't been tested in some cars anyway!