Another update - Have had 8 turrets made up now (minimum order)
The accuracy is spot on!.. although I have made it 1mm taller than it needs to be, but sanding it down is easy enough to do and it means I have sand it down the correct amount to make it a bottoming turret which is good.
The spline fit is perfect, very tight fit and it doesn't seem likely that the turret will be able to turn without turning the brass internal post on the scope (which wouldn't be good).
The finish is not fantastic - it's as the company described - a bit like an extra strong mint.
I think with a little bit of sanding and painting it'll look pretty good and at £2.50 you can't argue with the value for money.
I've attached a photo of it fitted to the scope, excuse the grubby marks around the top.
I'm going to have a play with spraying it black and highlighting the numbers and click marks in white. I suspect that an accomplished Airfix model maker / Wargames figure painter would be able to do an awesome job.