As said they use the smart charge system, you can test the alt in the same way as you would any other, but the smart charge bit is different You need a proper electrician/competent person not Halfords to look at it. The front screen takes massive power and only works when engine is running and would imagine it knows this by ECU and as car thinks it's not charging, no front screen. I suspect the clicking is a relay opening and closing. The car is obviously charging it just thinks it is not.

Those readings sound fine to me with it under load, the alt will charge at 15V+ direct because it's not internally regulated, the ECU regulates the voltage.

BTW you can test the voltage reading inside the car using the cars self diag.


Martin