The explosion damaged the toilet cistern and an air vent. There was nobody in the restroom at the time.
The remnants of a crudely made explosive device was found and taken away for forensic examination.
Investigators have viewed CCTV footage showing a man at the restaurant who is now wanted for questioning.
The man was last seen walking west along High Street. Police want to speak with him or anybody who might know his identity.
He is described as an 18 to 25-year-old of caucasian appearance, with shoulder-length brown hair.
He was last seen wearing a black T-shirt with a cartoon design on the front, blue denim jeans and a fawn-coloured jacket.
Anyone with information about the man's identity is being urged to contact Crime Stoppers.