Howard,
first things first. Do you know that this building is going to require Building Regulations Approval?
I am not sure of the situation in Scotland, but in England this can be achieved using a full set of plans ("Full Plans Approval") prior to commencing construction, or for minor domestic-type works you can use "Building Notices", where you take the risk of inspections as you go to confirm that you have built it properly (the risk being that you can be asked to take it all down again to do it right!).
Don't, whatever you do, attempt to take the latter route based only on what you glean from the forum.
It is a pity that you have ended up with a square building. 10m x 6.5m would have been much easier and would have been better looking.....never mind.
If this were my shed I would be trying very hard: 1/ to make it in timber and 2/ to have a clear-spanning roof (ie, no posts interrupting the floor area). Both are do-able.
You are going to need professionals on board, which will mean fees. Inevitably, this will require structural engineers calculations, whatever the construction method. Don't, whatever you do, let a structural engineer design it for you!!!!!! They generally think they can do it better than architects......but they can't!
I guess the next step is to see what you have PP for (ie........show us your approved drawings), and then find out about your preferred construction method and roof design (in principal only.......the big questions are do you want it to give you a clear span, and what is the approved roof covering?).
Don't you think 8m x 8m is being just a bit greedy??!!
Mike