I am running Shimano 105 10 speed on a Race Face Narrow-wide chainring.

Its started to make a noise like the indexing is out of line/not quite in gear. However, the gears are spot on.

Am I right in thinking its more likely the chain has stretched and is not quite sitting on the n/w chainring properly?

I’d check the indexing first.
It only has to be a fraction out to make a noise but still change gear ok.
Sometimes things settle in and it needs a little tweak.

Give it a tweak and see.

Is the noise from the front ring or the cassette?