Rowley Turton were so concerned about this issue that director Scott Gallacher actually raised Freedom of Information requests with the Department of Work & Pensions on this issue. 

In particular, Scott questioned whether the auto-enrolment contribution rules were compliant with a 1994 European Court of Justice ruling that excluding part-timers from pension schemes breached European law on equal pay.

The DWP informed us that "The Government Legal Department advises DWP on all aspects of legislation.The Department is satisfied that the auto-enrolment scheme is compliant with the ECJ ruling you cite.".

The government are not infallible but with Brexit around the corner the ECJ ruling might not be an issue for employers anyway.