Index trouble, worn cassette or bent derailleur?

ben@31

Hi after replacing a worn chain I noticed the new chain wasn't going onto the smallest sprocket so I thought I'll re-index the gears.
However now I'm having all sorts of trouble... I managed to re-index it so the chain goes onto the smallest cassette sprocket but the derailleur is moved so much it on the far limit of its adjustment. Then, if I change through the gears all the way from one end of the cassette to the other, some sprockets will be in index while others will be out. Then if I change from the big chainring to the small, some gears are that were in index will now be slightly out.
After fiddling for ages. I could get most of the cassette indexed but then on the second biggest sprocket ( second from top), you can see the chain badly skipping or wanting to jump off onto the next sprocket. This second biggest sprocket is the most troublesome by far, but it seems the sprockets either side of it are ok.
Is it a bent rear derailleur? I have came off not so long ago and this definitely bent the hanger, which was replaced. However as I've just replaced a worn chain and the indexing issue is effecting one sprocket more than others I'm wondering if the cassette is worn?
What would you replace? The derailleur cage or the cassette? Thanks.

wishitwasallflat

Personally I would take it to an lbs I trusted and ask them to check alignment for me. If you don't fancy that I would get both and fit the new hanger first. If neither sorts it out a spare hanger is good to have (usually cheap) and a cassette will get used in time so if you can carry the cost it's not a waste.

bungalballs

Before doing anything, have a quick look at the cable and check its running freely including barrel adjusters, bb guide etc. A draggy cable can cause all sorts of problems.

ben@31

It's a di2 groupset.
I had another look earlier with a home made plumb line ( bit of string tied to a padlock) and the cassette and the two jockey wheels weren't in a vertical line. I gave the cage a bit of a tug and the gears shift alright now.
Took the bike out for a quick spin and its alright but if I use a bit of force accelerating away from
A junction the chain slips on the front chain rings and came off. If I climb hard out the saddle the cassette makes all sorts of noises. Sounds like the old chain has worn the cassette and chainrings a bit.