If you employ someone who is subject to immigration control, they must have permission to be in the UK and to do the work in question.
You break the law if you employ someone aged 16 or over who is subject to immigration control and who works without the necessary permissions, including casual and seasonal workers.
Foreign students working in this country under the Seasonal Agricultural Workers Scheme will have a work card issued by the Home Office which shows their right to work in the UK.
More information can be obtained through http://www.ukba.homeoffice.gov.uk/
For specific visa issues contact the appropriate Embassy in London.
Immigation and Nationality Directorate
40 Wellesley Road
Telephone: 0870 606 7766
For work permits, seasonal agricultural workers scheme, highly skilled, and sector based schemes, e-mail email@example.com.
Telephone: 0114 207 4074