Mine's a 53 plate. I leave it on auto 90% of the time. Set it to "lo" and it produces cold air from the centre vents, with the fan at full power. I'll occasionally then override manually to reduce the fan speed. Likewise, at 18, if the car is really hot then the system will produce cold air with full fan initially and then reduce fan power as required. Generally it settles to 2 or 3 bars on the display. That's what it did this afternoon, with air temp of around 22 degree and the car having been standing in the sun all morning.
If yours in doing likewise then I'm guessing something's fecked, but I'm afraid I can't give any more suggestions as to what that might be. I understand that auto will direct air as necessary btw - certainly it pushes warm air to the footwells in winter quite nicely. HIH - Mike.