Thanks Maydup. I misinterpreted your point.
buildersmate is very good on this topic so maybe the difference in our answers shows I was perhaps a little soft as a manager, but I think I would just let the employee knew I knew and leave it at that, unless the employee's record was already a cause for concern and then I'd probably take formal action but on grounds of capability since it'd be very difficult to prove they weren't genuinely ill in the morning.
Anyway, is this scenario any different to someone who works full time for one employer ringing in sick in the morning and then turning up in the afternoon saying they feel well enough to work now after resting? I doubt an employer would give a warning then.