Mistake, Duress, Undue Influence and Illegality

A farmer agrees to sell 500 tonnes of grain stored in a warehouse to a food manufacturer for £200,000. Unknown to both parties, the grain had been destroyed by flooding two days before the contract was signed. The food manufacturer has already paid a deposit of £50,000 and now seeks your advice on the legal position. The contract makes no reference to the condition of the grain and contains no force majeure clause. Both parties genuinely believed the grain existed at the time of contracting.