progress report- fished the tank out tonight and it's looking pretty damn good. It's been soaking in one of those storage tubs you get from the el-cheapo stores with about 2litres of white king bleach in enough water to cover the tank. It's taken till now (5 days?) to start looking really good, and I did scrape and sand the tank a bit before I started soaking.
I'll post a pic when I get the chance.
BTW, all the fittings were removed before I dunked it.