Hi

Only thing left of concern is the span - we want to put down an
octagonal deck - spacing uprights every 4 or 5 feet at the corners is no
problem, but there will be a span of over 10' across the deck. Does this
mean I'll have to put a central support in or do you think the joist
structure is enough ?

I suppose it depends on the structure. 4 x 2 or 6 x 3?

Floors in houses span much more than 10' without central supports don't
they. Personally I'd play it safe for what it would cost in time, effort &
money. That way it won't flex.

Dave