Thanks for the comments.
It's definately a functioning sight, I've attached a pic and you can see the body of the sight moves left to right depending on the direction of the set screws on either side - as you can see it's hard to the left.
This is the front of the SUSAT, initially you might think "hang on, that's pointing to the right". But of course the bottom of the sight is static onto the rail, it's the lenses and casing that physically moves left/right.
But yes @Adolf Hamster I did try the other way, because nor do I trust the markings on a replica sight and I thought maybe a prism arrangement was having some opposing effect, or I was just being an idiot. But it went right out of sight the other way. Also, if you pin the SUSAT down on the bench, loosen off both screws then look through it and move it left to right, you can see the reticule moving the right way, so if I could move it further left then I could get it on-target but I'm not going to start taking metal off.
So I reckon the reticule is just off-centre - either been dropped or just badly assembled at manufacture.
I think it's time for an email to the seller and ask if they'll just replace it. I'll let everyone know how I get on.
All the best!
Jock