Does this mean therefore, that whilst I was flying with other syndicate members (i.e. not instructors) I was technically a passenger, even though I was 'sole manipulator of the controls'?
Yes, if you do not already have differences training you cannot legally be P1 in the aircraft. If you have a tail wheel sign off and simply want a check-out under the syndicate rules, then you can log it as P1. Technically you have to agree which of you is to be PIC, but I don't imagine there would be any argument in case of a check-out flight.