It's been a while - more planning and then finding the time to get the work done.
As you will see in the attached photos, I cleared the ground under the obs and put down weed matting. I then put the joists in and made the floor from marine grade ply lined up with the joists so that it doesn't creek when you walk on it
The walls went up on rather a windy day which was fun but with five of us not too bad. I was able to bolt it all down through the flooring into the joists then made up an angle iron frame for the top of the walls.
Today I finally got the roof on and it works a treat - rolls nicely with one hand.
Next up are tie down points so that the next big wind doesn't see the roof sailing off into the distance - I've put ratchet straps over it for now, just in case.
After that will be shuttering around the roof/top of wall area to stop horizontal rain getting in.