Well I got there in the end, not sure how I did it though other than a painfully low speed and a bit at a time, it was still 'singing' a bit but finally managed to get a chatter free finish.
I was beginning to think I had a machine problem even though could find no movement in the headstock but I did notice play in the drive dog (it is a lathe/mill)
As suggested earlier, when I came to part off the heavier cut tended to minimise the chatter, you just have to be brave and keep winding the tool in.
Changed to cast iron as well which machined beautifully although parting off did not seem too different to MS.
Thanks for all the replies.
Bill D.