Hi Peter, I don't know about portable clocking in clocks - can't see how one would work. If it's portable it can be fiddled or the other alternative is to drive around with it in which case you're checking up on your staff anyway. Maybe I'm missing something here?
As an alternative, there are timekeeping/voicemail systems on the market. Your cleaners dial a number and send their personal code down the line. They are used for clocking in and out, and have additional H&S benefits on single cleaner sites. They can also be linked to your payroll.
Try
www.expolink.co.uk and look at their StaffCheck system.
Hope that helps.
Musicman