Thanks all for the feedback. I really didn't mean to start the usual argument about oil viscosities and hoped I had worded the initial post to avoid that! That said, I obviously looked into this over the course of a very expensive year in terms of oil consumption. My views are identical to what scottyf describes and I really just went for the 10w60 in desperation to keep consumption down even though I knew it would offer less protection. FYI, I'd also tried 5w50 and 10w50 as a half-way house before going 10w60. As I now know, grade was not teh issue.
Regarding where the oil is actually going, I have spoken to the Gary at Bianco Auto Dev who I have always trusted absolutely on my Alfas since 1995. I will pop into him as soon as I can and he will do a diagnostics test and check the plugs to see if the loss relates to a particular cylinder. From discussions with him and others, 3 most likely causes seem to be:
Valve stem seals
I'll keep you updated, hoping I find out and it's economical to repair. I dread having to take the head off...