"If you have a joint account, then both you and your partner have access to the account, regardless of whether only one of you pays into it. If your relationship ends, the money will belong to both of you. However, if one of you didn't use the account at all, for example, you didn't pay any money in or take any out, it may be difficult to claim that you have any right to it.

If the account is in joint names, on the death of one partner, the other partner becomes entitled to the balance and can continue to have unlimited access to the account. However, a proportion of the balance will be taken into account when calculating the value of the estate of the person who has died."
Goddamnit, state. Keep your paws off the joint account.