wot bus goes from city centre to rotheram(magna, or as close as?)

No 69 I think. Don't know where you catch it though.

Yep number 69 is the most frequent.

Catch from Sheffield Interchange, Wicker, Brightside Lane, Meadowhall Interchange.

Are various other buses to Rotherham, mostly from interchange

The 69 goes from the bottom of Commercial street, just above Ponds Forge opposite the Fitzalan square tram stop.