I'm afraid it maybe worse than being sued, it is likely that you may have committed an offence under the Dangerous Dogs Act 1991.
Here's the law.
3 Keeping dogs under proper control (1) If a dog is dangerously out of control in a public place�
(a) the owner; and
(b) if different, the person for the time being in charge of the dog,
is guilty of an offence, or, if the dog while so out of control injures any person, an aggravated offence, under this subsection.
Here's the punishment
4) A person guilty of an offence under subsection (1) or (3) above other than an aggravated offence is liable on summary conviction to imprisonment for a term not exceeding six months or a fine not exceeding level 5 on the standard scale or both; and a person guilty of an aggravated offence under either of those subsections is liable�
(a) on summary conviction, to imprisonment for a term not exceeding six months or a fine not exceeding the statutory maximum or both;
(b) on conviction on indictment, to imprisonment for a term not exceeding two years or a fine or both.
They can also order your dog to be destroyed.