Ultra valve leaking from spindle

[diy hacker]

Keen amatuer that I am I decided to have a go at changing the cartridge in my shower when the old one failed. I had a spare as I got one when I had the valve fitted. The cartridge is as a SC50T32 I followed the video on the Ultra technical site l and it all went smoothly but I now have a constant weep from the spindle. I have removed and reseated the spindle sealing screw as in the video several times but it still onstantly weeps, am I missing something or could it just be a faulty valve?! Any advice appreciated