It has taken about 60 man hours but finally all the bottom paint is off.
Step one - Internalional Interstrip and scraping
Step two - Hempel paint remover and scraping to get the tie-coat off
Step three - sanding - sanding - sanding
And the problem?
Now all the paint is off it is clear that where the bootstrap had been painted there was no tie-coat and the gelcoat has deteriorated and cracked. Were it was protected by the tie-coat or topsides paint it is fine. Best of all there are no signs of osmosis blisters.