It is legal to use covert cameras and/or listening devices to record your staff IF you have very good reason to believe a crime is being committed AND you use it only as long as is necessary to get the evidence.
If you haven't obtained the evidence within a reasonable time (how long is reasonable?) you must stop.
You CANNOT record staff in toilets and wash rooms except in extreme circumstances.
http://www.ico.gov.uk...e_html/appendix3.html
I suggest you keep a log detailing your reasons for installing the equipment, the date it is installed, where it is and what it is recording, the date it is removed and your reasons for not informing the staff.