That’s all good Guy, I don’t know if I was being a dummy but you helped me fix the problem!
I must have not been powering the MS 6000 on with the V-TAPE button first, when I did that, then switched to S-Video the sound came through the Scart as we would expect. I imagine previously I was powering on with the TV button then changing over to V-TAPE, all seems so obvious when things start working. Thanks so much for your help, very happy to have this working, now my N64 can look decent quality through the S-Video with nice B&O speakers.
Admin is welcome to close the thread of necessary. Thanks!