iv recently changed the gearbox oil in my bike and what seemed to be the right amount came out but was very dirty,iv put new in and cleaned everything up then came back to the bike a few days later and its slowly leaking from the underside of the engine (I think the center gasket?) is this common on these bikes? there was definitely no leak in the 1st place.

No, not common. Only issue has been those bashing the skidplate really hard and it can push the pad up hard into the clutch side cover(main one) and distort it, breaking the seal.

I would inspect it, and clean it really well to see if it wets itself there, possibly.