Update: After a lot of thought about this problem area, and my shaving generally, i purchased a very fine Merkur Progress 510 long handle from Connaught and have been enjoying (carefully) its potential.
After nearly 2 weeks ownership and about a dozen shaves i am happy that a setting of about 2 (with an Astra SP blade) equates roughly to the shave i get from my EJ89 and if i turn the Progress up to 3, then i get a much closer shave, but it can be a little "nippy" at that setting if i am careless, so max concentration is required. Setting 3 seems to make better work of the original problem area under the chin and around the neckline first pass, so am very pleased about that.
Am still getting to grips with the Progress as it is a heaver razor and the shaving angle is a little different, but am generally very pleased with it and think it will serve me well. A pleasant discovery is that if i turn it right down to the zero ("+") then it gives a very light shave almost comparable to a cartridge razor and ideal for those afternoon / early evening touch ups (the old 5 o'clock shadow).
This is important for me, as 12 hours after DE shaving i have quite tough sandpaper-like micro stubble around the neck, which is rough enough to bobble up the material on shirt and fleece collars - something i have had an issue with my whole adult life, requiring some sort of afternoon / early evening quick shave usually with a cartridge or even electric shaver.
I have noticed there is quite some play in the blade and the head when its fully unwound for blade changing and the process of tightening it up can cause the blade and head to skew slightly and can give either a slight slant, or more blade out of one side of the head than the other.
After a bit of playing, i think the answer seems to be to centre the blade and head, then carefully clamp the head down against the spring using thumb on top cap and two fingers under bottom plate - this allows the tightening screw to be tightened easily and ensures the blade and top cap are always aligned as intended.