Diesel 1.9 hatch back

I have an interesting problem.  When my car has been in sunshine and I turn the airconditioning on it works fine however, if the car has been in the shade or the outside temperature has less than 10-15 degrees then all I get is hot air and I mean hot. There is no way of regulating it at all and the only way I get any respite is to turn the heater off completely.  Even if I stop the car then start it again the airconditioning doesn't kick in.

I'm totally flumoxed

Can anyone help, there are a number of airconditioning sensors I can replace but which one is the culprit, perhaps someone else has had the same problem and can impart some knowledge on the subject.

Thanks

Robert