Okay, some further progress but also one large step backwards!
I have been building the three steam pinnaces for Invincible and used the plans from "The Anatomy of the Ship - Dreadnought" as the boats are drawn to 1/96 scale.
All was okay until I went to fix the outer pinnaces on the aft flying deck. Once fitted there wasn't room for the inner two boats. Much checking of measurements ensured and on checking the pinnaces shown on the Sambrook plans and the "Original Plans" (Battlecrusiers by Roberts) showed they are actually much slimmer than those shown in Dreadnought!
Further checking shows that the Dreadnought ones are 11 feet wide and the Invincible ones are 9 feet wide. All this adds up to 1/2 inch less width between the outer boats.
There is a surviving 50 foot steam pinnace at Portsmouth and the web gives dimensions as 50 feet long buy 9 feet wide. Interestingly the specific pinnace was actually mounted on Inflexible (Invincible's sister) for a period of time so I think that confirms the authenticity and measurements.
I can only conclude the Dreadnought plans are either inaccurate or perhaps were distorted during printing giving a wider beam.
Basically the three steam pinnaces are now junk and have to be completely re-made. Measure three times and check all available sources before cutting!!!