Hello and thanks for using Just Answer.
My name is ***** ***** am happy to assist you with your enquiry.
You are quite correct, in that your Father is classed as owning half the value of the joint account.
If applicable, you would have to prove that more than 50% of the £40,000 belongs to you to argue otherwise. You would need documentary evidence to prove this.
I hope this assists you and clarified the legal position.