10 speed chain skipping on 9 speed cassette

This feels like a new chain on a worn cassette problem. The fix is to replace the cassette, and soon before your chain suffers accelerated wear.

Assuming you have 9 speed shifter and rear mech still, the slightly narrower chain won't really make any difference while pedalling, and shifts might be slightly sloppier.

I run a 10 speed chain on a 9 speed cassette without issue, because its what I had at the time.

I have solved the problem. Used equation from park tool site for chain length. Turns out my chain was too long and probably this exacerbated by unscrewing b screw. Removed a full link outer to outer and its now working much better and almost no slipping.