yep . that is correct . the alt wasn't puting the right power in. hence battery was discharging slowly . therefore not putting out enough for a strong enough spark . hence car wouldn't boost properly . the car in question was c- reg phase 1 .
fitted new alt , no battery light coming on at higher revs . boosting fine .
you will find that the gt turbo has a higher amperage rating than a non turbo 5 . due to extra power drain that the gt turbo produces .
i had the alt checked at lsuk in yeovil . they told me that it was the amperage output that was weak . but it was still charging . but wouldn't be enough to supply what the car required .