Martha Jones - a poor black imitation of Liz Shaw?

Absolutely not!! The only thing Martha and Liz had in common was that they were both professional, independent women. Liz was a scientist and Martha was a trainee doctor but Martha was an incredible character - she was fun and great to watch and she was a breath of fresh air after Rose who was so clingy with the Doctor. Liz was good too - but sometimes she was so stiff it was hard to differentiate between her and the Autons! Martha was a much more memorable character and more likeable.