Initially I thought that the contention that parliamentary democracy was part of the superstructure built on the material basis of society wasn't necessarily true. Obviously Marx was long dead by the time workers were enfranchised in the UK so he can't help, but I think it probably is true that advanced capitalism requires some form of democracy for stability because with the development of the productive forces with increasing division of labour and therefore education, one inevitably gets the ability to think for one's self and not be dictated to. Dictatorships generally exist in societies that have not fully developed fully fledged capitalism, i.e. mostly peasants instead of mostly working class.

These are quite interesting on capitalism and democracy.
http://www.spgb.org.uk/Q-democanddictat.htm
http://www.spgb.org.uk/Q-parliament.htm
http://www.spgb.org.uk/Q-marxanddictat.htm