Chinese citizens can transit/stay in Hong Kong for 7 days without an entry permit if they have the return fight tickets. If they wish to stay over seven days, they must apply for Entry Permit at the Chinese Embassy or Consulates in U.K.
Detailed instructions for application for a Hong Kong visa or Entry Permit are listed in "Application for an Entry Visa for Hong Kong".
For more information about Hong Kong Visa Applications, please visit:
http://www.info.gov.hk/immd  
Tel: 00852-28246111; Fax: 00852-28777711