Once the ascorbic acid arrived, I followed your instructions. Unfortunately despite a high concentration of ascorbate (1lb in 1000 galloons) and sequestrant and vigorous wire brushing, the brown stains did not completely come off even after 48 hrs of recirculation. Of note there was quite a lot of a brown precipitate on the floor of the pool.
I emptied the pool and applied ascorbic acid powder directly to the brown stains (having dampened them with a sponge). The next morning there were green stains in the treated areas (presumably iron ascorbate) which were removable to some extent by washing with tap water (pH 7), the remainder coming off with a gentle power wash. A quick google failed to reveal the physical characteristics of ferrous/ic ascorbate. There was no damage to the plaster.
The pool surface is now free of iron stains and refilled with iron free water! Thanks for the help:-)