HEVAC Issue 97 model HSE

I have a strange issue with my HEVAC, the book synbol appears after approx 5 minutes of running but only when the Air Con is switched OFF, if I swith the Air Con on the book symbol never appears despite the Air Con not kicking in. If I turn the ignition on but dont start the vehicle with the Air Con switched off, no book appears. I have had it in a local garage who cleared a list of faults but couldnt diagnose the problem, I think that despite me telling them the fault only appears with the air con off, they checked for the fault with the air con on, in the end I gave up and will try somewhere else for diagnosis but thought I would run it past this forum to see if anyone can shed any light on this issue. The Air Con system was regassed 12 months ago as the evaporator had corroded through and had to be replaced.

Any help grately appreciated