Was does it mean by this horse could have settled better? Surely that means gelding it? But I'm loathe to geld such a good horse. I tried with the visor as you can see but it had no effect. Any other suggestions?

If it's a group winner whatever you do DON'T GELD IT! Unless it's form tails off or when you watch it race it is visibly not settling, e.g. jockey restrains it then the horse pulls to the front and jockey restrains it again etc. If that had happened then the horse wouldn't have won like that.

I tend to ignore this unless the horse really doesn't settle as above or if the form reads 'Not settle' more than once when the horse hasn't got a red star next to it's name. Nearly all of my NH horses 'don't settle' but they have no problem winning Gold Cups, Grand Nationals, King Georges etc. and some of my flat horses also 'don't settle' but lots have had no problem winning multiple feature races.

I think it's an error in the game.
My horses do it all the time, but, like SuperCat says "Don't geld it" this problem as been ongoing since SO4 launched last year.

as ESA would say....GELD IT!

I'll try to remember that when I run stamina horses over short distances!