Posted: Yesterday at 23:29
You probably have the right to rescind the contract if the goods supplied do not comply with the description of what was sold to you. The "samples" on display indicate that the shed would be properly treated inside and out, and clearly this has not been the case. (Misrepresentation Act used to cover this - not sure if it hasn't been superseded, though). Therefore, you could expect a replacement or a full refund.
The Trades Description Act is of no direct use to you, since it is a criminal matter, enforceable by the Trading Standards Department at your local council. They will look to see if the faults (preservative, size etc) are "false to a material degree". If they choose to take action, it will strengthen your case against the company (if they are successful) but it will not, in itself, give you the solution that you seek.
Better, I suggest, to continue in the civil action: goods must be as described, fit for purpose etc. It seems that you have a strong case. But you may need to be persistent to get the result you want and deserve. good luck!