In my head though I am leaning towards the Altberg boots as they appear to be half the weight and I've got a knee issue so the less clunky the better really. This probably puts the option of the defenders out as well as they look like beasts but appreciate the suggestion.
The weights you've looked cannot be compared with each other - weight per boot against weight per pair.
The weight of the Altberg Tethera is 736gms per boot (Size 9) or
1472gms per pair against the weight of the Scarpa Delta GTX of
1520gms per pair (Size EU 42/UK
so in reality there's not much in it and the Scarpa Delta is only 58gms
per pair heavier than the Tethera (or slightly over an ounce per boot in English money).
By comparison the Altberg Defender is 741gms per boot (Size 9) or
1482gms per pair so they're only 10 gms per pair heavier than the Tethera and in reality not that much of a beast but their major advantage is that they definitely are much cheaper to replace when they inevitably wear out with your daily dog walking use.