Thank you everyone for the quick and helpful replies. One of the things that I love about the narrowboat community is the way everyone seems to help each other
It seems I may not have explained myself very well. The readings that vary from 9.7mm to 6.4mm are all taken along the bottom on the hull. There seems to be a steady, but not necessarily consistent, reduction in hull thickness moving from bow to stern. It would appear that the original thickness of the baseplate must have been 10mm, so I'm concerned that the wear is excessive around the stern. The boat was built in 1994, so this seems like a lot of wear in that many years.
When the boat was surveyed last year, the surveyor wasn't concerned by these readings, so maybe I'm just being paranoid.
Thanks again,
Dom.