Keith
I have grave reservations about sticking my nose in on this one, but for the first time ever I could find myself at odds with something you have posted.
Your last sentence was “The addition of anhedral both on the wings and tailplane, reduced the lateral stability, thereby restoring a reasonable degree of roll response.”
I can only agree with this if the roll response you quote is that due to a lateral gust. If it is the roll response due to aileron deflection that you mean, then I disagree.
The rate of roll that happens following aileron deflection on any aircraft has much more to do with the roll damping of its wing rather than any lateral stability it may possess. Static lateral stability of the sort produced by dihedral ONLY produces a stable righting moment (that opposes ailerons) if sideslip also develops from the side of the down going wing.
It is the need to obtain a reasonable (as opposed to huge) response to a lateral gust that leads people to employ anhedral. Without it an aircraft with a high wing and low CG, especially if the wing is also swept, will have a terrible (huge) tendency to roll when hit by a lateral gust. So whether it is a 146 (where you are concerned about passenger comfort as well as controllability on a gusty approach) or a Harrier (where aiming steadiness and low level ride are the considerations) the answer is to include a rolling moment due to sideslip that has the opposite arithmetic sign to that coming from sweep plus high wing and low CG. This means anhedral.
You can even use huge lateral stabilty to generate high roll rates. Nearly 40 years ago as a very new tp just posted to Aero Flight I was given the SB5 (Lightning look alike with a tiny donk and a fixed gear) to fly on an open day. All of the experienced guys having declined to be seen dead in the device picked rather more manoeuvrable mounts (like the FD2 or the HP115) On my first flight I found why: the stick forces were huge, the rate of roll from aileron was negligible (thanks to them being unpowered, very close to the fuselage and being almost too heavy to deflect). But the monster had a sweep of 60deg so this huge lateral stability enabled a modest facsimile of a twinkle roll to be produced by kicking the rudder and making deliberate use of the huge rolling moment due to sideslip.
Regards
John