No one really knows for sure, but to my knowledge simply because Hugh played on every record does not mean that Alec never played a note on them. It's just Hugh is better at writting bass lines, but to me, the bass line for say, Wild in the Streets, '99 in the shade, Sleep when I'm dead is something that Alec could have played and written since he comes from a very 'stones' background. But no one will ever really know for sure...