1. Dear Martin, I regret to say that this clause is fully enforceable and you will have to negotiate if you wish to be excluded from its remit, or else pay the 20% penalty. This type of clause is very common in hiring or work arrangements and the courts have held that this type of penalty clause is enforceable. Essentially, the parties are taken to have freely negotiated this type of contract clause in their agreement and the courts will enforce an agreement of this nature. They won't strike it down as being unfair or unjust.
2. As this is a commercial agreement, the courts are loathe to interfere in this type of contract. So, the courts basically leave things to the parties to decide what terms they wish to be bound by. They will also aid their enforcement.