The alt could do with a load test, not just a meter on it.
The heated front screen uses far more electrickery than the rear, and so if the output drops under driving, it may well either cut the front screen or disable it, on older cars/fords the front screen relay was fed and ebergised one side by the warning light for the battery and so would ONLY work once the engine was running/charging.
I would take to a proper sparky, quite surprised halfords knew what a battery was, must be getting better!!
tabetha