NavList:

K McGrath you wrote: I also have a Hughes 6B sometimes refered to as a flying boat sextant. I haven't seen much written about these but I assume they would have been used mainly for taking horizontal angles to estimate take of distances available. Presumably something like a Sunderland would also have carried a MK IX. The 6B is a surprisingly accurate and nice instrument to use.

Re: The Hughes Flying Boat Sextant.  I think you’ll find that the 6B is followed by 177.  I.e., 6B/177.  That is the RAF Stores Reference for the device.  All RAF navigation equipment has a 6B section number e.g. 6B/472 is a Navigator’s Pencil Box, 6B/349 is a Navigator’s Perspex Rule 0-140nm on ½ Million Scale.  They probably even had a 6B number for a Red/blue Pencil Crayon.  Hughes and Son appear to have simply called the device the 3 ½” Radius Marine Sextant.  I’ve also seen it described as a lifeboat sextant.
Yes, you could use the sextant horizontally for finding your exact position or recording soundings close to the shore or to check if your anchor is dragging, but it was also used vertically for celestial navigation.  So long as you had a sea horizon visible, a marine sextant was more accurate than a bubble sextant, because you didn’t have to worry about acceleration errors.  The dip tables extend quite high, but the RAF advised using dip=1.063 square root of height above sea level in feet.  Below 1,000’ you were allowed to forget about 1.063 (although at 1000’ that would equate to 2’error).  DaveP