Have a look at http://www.cjsonline.gov.uk/juror/expenses/index.html

Explains what expenses are claimable. There is a loss of earnings clause and looks as if the employer needs to complete some forms for this. The juror needs to submit these forms on the first day they attend, so they might need to get a copy of they haven't already been sent one.

I did JS about 12 years ago, but worked for a major company and they paid me for my time.