Having inspected both my seating dies stems/rods, I conclude I must amend my die choice for each bullet type.

I have used the RCBS seating die with the 160gn RN & the Redding seating die with the 140 gn SST bullet.
WRONG!
The Redding die has a much broader seating cone, where the RCBS is narrower & more designed for 'pointy' bullets.

So, resetting the most suitable seating dies for each bullet choice is my first move to improve matters.
I also ordered a Lyman VLD chamfer tool...it has a 22deg. angle chamfer and will probably aid the flat base of the bullet into case neck. It'll mean I don't have to use the expander die which 'works' the case neck rather more than I like.

Taking your advice Daryl, I've already arranged to borrow a friends Hornady 6.5x55 seating die. His die has the supporting sliding collar aiding bullet alignment.
Looking at the die, from his de-luxe set, the seating rod is 'pointy' and the rather blunt bullet nose of the 160gn RN sits on the edge of the seater rather than 'in' it. The collet works in terms of bullet alignment.

I'm now told there is a set of Hornady de-luxe dies available as a stock item for MSch 6.5x54..now they would seem good if the seater is more rounded for the 160gn bullet.
I'm not sure how true this is though, and finding a set in stock with a dealer (to look at the seater) may prove interesting!