Thanks Guys. Sorry about the delay in replying. Have been away for a wedding for a few days.

I will try and figure out how to post a sketch or two tonight but until then...

A compression band around the top plate was the original plan with maybe a couple of secondary bands over the top of the lining boards and anchored to the hip and jack rafters to prevent spreading.

I am not a huge fan of tension cables and definitely not of the timber framing option. I use timbers for their structural capabilities, never just for a purely decorative purpose. What is the point of all that extra framing when a king pin, rafters and a couple tension ties would suffice.
Inspiration for the frame came from Steve Chappels new compound framing book which shows an enormous 56ft clear span hammer-beam octagon of a similarly low pitch. I don't personal know Steve but I get the strong impression that he is not the sort to lace his frames with steel. The design and timber sizing for my frame is simply the result of necessity. What can I put together with what is in the timber stack without having to resort to cutting or buy more timbers. I should point out here that the timbers I plan to use are significantly more sturdy than those specified by the engineer for the initial conventionally framed structure.
On the engineering side of things. Design loads for the structure are as follows
Dead load for roof 90kg/m2 (about 18lb/sqft)
Wind load 1.3Kpa for the roofs cross section which works out at something like 14lb/sqft live load.
This works out at something a little over 30lbs/sqft maximum total design load.
I don't know the exact numbers for timber properties. We grade timber very differently here to the US and it would take me a while to figure out how to convert it all. However the timber I am using for the majority of the frame is said to have very similar properties and characteristics of dense western larch.

I hope all this helps and look forward to your replies.
I will do my best to get some images of sketches online in the next 24hrs.
Cheers