Turntable sibilance

Okay turntable Yodas, what is the primary cause of sibilance in vinyl replay? Obviously some records are badly cut & it's in the grooves, but I seem to have an inordinate amount of it.

I have a Shelter 501 mk 11 cartridge on a Rega RB 250 arm on a Nottingham Analogue Studios Horizon table. Running vtf at 1.8, but changing it in either direction makes negligible difference. Excuse my ignorance, but how do you change the vta on these arms, & might that be the issue? Setup was done by reputable hi fi store, spirit level says table is flat. The green sliding horizontal tracking adjuster doesn't seem to much either.

Any suggestions would be received gratefully.