A comment from a 'leftie', ( as you asked for one ).
From reading the entire article, it would appear that the cards are generated from a database used by both Tories & Labour ( "Experian, the data management company, confirmed that both Labour and the Conservatives use its Mosaic database, which divides voters into 67 groups. The databases can use anonymised hospital statistics, including postcodes and the diagnoses of patients, to identify the likely addresses of those with particular illnesses" ). This is not to suggest this is a correct use of such information, I personally think it isn't, but it would appear that there is no breach of the DP Act as the information seems to be some sort of 'reverse engineering' process based on publicly, ( commercially ), available databases. Smells too much of Mandy's cynical dirty tricks department.
I bet the Tories are just thankful that Labour used this data first.