I can hear it now - Haddock shouting "measure twice and cut/drill once".
When laying out the base I used the tank model to get the correct size for the hut - but forgot I was adding bar-armour on each side, so the hut is not wide enough for the tank. Have now re-drilled for 2 of the posts and am making changes to the roof cross bars, so that they now fit. If nothing else it will have given you lot a laugh at my expense.