Wow - thank you all for such a quick and detailed response.
First - a confession - I did not employ the services of a surveyor, being an engineer myself, so did all the visual inspection and measurements. Probably not my best call to date but there we are. Subsequent discussion with a surveyor suggested this was quite a common occurence - plenty of thickness left!
A little more history - first, I understand the boat has been moored for its entire life at an end-of-garden morring near Kidderminster - this would make it River Severn I assume. It has cruised of course but the majority of the time it will have been 'at home'. I also assume it will have had a shoreline connected for most of that time - no solar panels so shore power only. I have yet to see a GI on the boat. My thoughts at this stage are a combination of the presence of the shoreline and the absence of anodes through the centre section of the hull - but this is supposition, hence my shout out.
The boat hasn't been lifted for 5 years, since it was shot blast and received 2-pack epoxy to the hull sides - this has stood up well but has a small number of breakthrough points from physical damage - nothing serious and certainly nothing like the attack to the baseplate. The edge of the chine is the first place to show any colour, with rusty looking 'bubbles' - picture attached (I hope).
Underneath is a different story as the attached pictures will show. Thickness measurements showed 9.5 to 10mm generally. The depressions don't look like impact damage or distrortion, simply hollowed out areas where the [late is thinned. As you can see, they are smooth sided and look just like finger tip and thumb prints in rolled out pastry. The crust was fairly thick in places, maybe 3 or 4mm and was black against the steel. You can see where I have scraped off varying size sections of it. The pictures are obviously upside down due to my position!
I note the comments above on MIC and hesitate to acknowledge the orange growths I saw.
Your comments are invaluable - thanks.