Should I include my C in AS physics on my CV?

it depends on what jobs you’re applying for. if it is or could be relevant, definitely. for student jobs like cafes or whatnot i’d leave it off.

It's for science graduate jobs that don't require physics knowledge.

It's very minor, but it looks like a stain next to my AAA at A-level and first class result in 1st and 2nd year of uni.

Stain? Employers really don't select like that.

Could you please explain what you mean?

You are presumably applying for an internship or a graduate role. Employers select positively on attributes they positively want. Academically, that might be a degree classification, and for some employers it might might also include a sustained high level of academic ability, so include A level results. But employers do not 'mark' negatively, you don't lose points for having a weak additional qualification. So whatever their interest in your academic ability, AAAD is better than AAA, if irrelevantly better.