ICT SLB, thanks for you assurance. Although I flew the 1-11 and in a very small way was part of the 3-11 concept, my information comes from being very much closer to the 146 project.
The reference document was “A summary of the work and conclusions arising from the 1972/73 joint HMG/BAC funded programme of R/STOL studies.” 18th July 1974.
Surprisingly the STOL project in the studies always had 4 engines, whereas the RTOL/conventional aircraft (CTOL) evolved from a high wing, 2 eng aircraft to the low wing 2/3 eng ‘Airbus / 3-11’ idea.
Whilst my information does not refute an association with the HS 681, I suggest that some of the essential components were not the same as the 146, e.g. fuselage diameter, wing sweep, and pontoon main gear.
Many of these aspects were discussed in the research document. In particular, the ‘blown flap’ (internal and external) aspects of STOL performance and gear shape / location. Whilst the 146 might have a very small ‘blown flap’ component of lift, it is miniscule in comparison to what was considered in the research. Some of the concerns were with pitch control, this required very large tail surfaces, possibly powered controls and leading edge flaps.
The simpler, lower risk concepts were followed, which I suggest are considerations in the choice of wing location – 146 manual controls, no leading edge flaps.
A simpler fuel system was another advantage of the high wing; emergency gravity feed to under wing engines and no fuel pipes in the cabin – unfortunately the 146 failed with the second aspect.