I’m looking at it and can’t tell. Does the hanger look slightly bent?
No. But that limit screw 
LBS was the last one to service this bike…
I think that’s the B-Limit screw - maybe you’ve a large cassette on with a medium/short cage mech?
Bent gear hangers enough to throw off the indexing are almost never visibly bent, but still throw things off, a gear hanger alignment tool is well worth having (access too)
Looks fairly straight. Why would it be bent? Have you crashed?
I think the gap to the cassette narrows as it moves away from the skewer.
I also get more friction through the drivetrain when I’m in the 11/12 cogs. Trying to figure out why.
difficult to tell from one photo but I think it might be slightly hanging to the right (of the pic). if it is slightly out try and bend it back a little and see what happens - shouldn’t take too much pressure to do that but don’t be too forceful!
Straigthen the hanger, seriously, it’s sometimes worth doing to brand new hangers, the tool is not that cheap though, we lend ours out pretty often.
Looks in that photo as though the deraileur might be pulling the chain slightly outboard (away from the wheel) as it comes off the sprocket. Not sure though. My son’s MTB came out the box with a slightly bent hanger and the effect of that was I could only get part of the cassette indexed. If the larger sprokets were spot on, then it wouldn’t change accurately on the smaller ones. Might that be the case with yours? On his bike I could see that the cage wasn’t perfectly in line. Hard to see, though, and I guess if yours is bent it’s even less than his since you haven’t mentioned actually missing shifts/dropping back down. I think I found it easiest to see looking down from above. You’re looking for the jockey wheels lining up - or not - with the sprocket. After becoming convinced it really wasn’t me I took it to LBS and they checked and straightened it in 10 mins for £10. He then went and stacked it later the same day and that was another £10…
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I once wrangled with indexing for hours before realising the problem was a loose rear derailleur hanger. Where it attaches to the frame, the hex bolts had worked loose and it was able to move a little.
Not saying this is wrong on your bike, just that it’s a thing that can go wrong
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I had a similar situation a few years ago. Really irritating noise when pedalling. Looked at everything. Spent hours, then eventually realised the tiny hex bolts on the hanger were loose!
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I’ve had that on this bike in the past. I check those bolts once a month pretty regularly when putting the bike on and off the turbo.
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Thanks for advice. checked the limit screws and they seem to be ok, chain hasn’t dropped off so not likely to be much out if at all.
Something definitely feels out somewhere and given my suspicion that it’s the hanger I’ve just decided to try replacing it. The cost of a new hanger is significantly less than the the tool to straighten the one I’ve got so might as well give it a go.
use a large adjustable spanner - it will grip the hangar well and has a long enough handle for leverage. done that method a few times
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I’ve had brand new hangers that need straightening though!