A JUDGE said yesterday it was 'quite extraordinary' that an undercover policeman took part in a criminal plan which led to an assault. Judge Surman made his comment in the District Court during a case in which he jailed construction worker So Chi-ming for 20 months. So, 29, admitted conspiracy to cause grievous bodily harm and assault occasioning actual bodily harm. The conspiracy involved So, other men and an undercover policeman plotting an attack on a couple beginning with ramming their car. Judge Surman said: 'It is quite extraordinary for an undercover agent to get involved in a deliberate bumping of another car with the idea of a fracas . . . inflicted on an unfortunate man,' he said. The court was told that two co-accused, Wong Lee-how, 33, and Wong Hon-chung, 26, had absconded. Prosecutor Agnes Chan said on August 14, 1994, the group, including undercover policeman Ricky Lo, discussed a plan to assault a Chinese couple. On August 18, at about 8.20 pm, the policeman drove his car into the male victim's Mercedes Benz at the junction of the Olympic Avenue and Hang Wan Road, Kowloon City. The men then got out of the car. The policeman swore at the intended victim, and then returned to the car while the other men attacked him. The assault went on for two to three minutes. Raymond Yu, for So, said he was not a 'professional troublemaker or die-hard criminal' and had a previous good record. Judge Surman said he had taken into account So's guilty plea and his remorseful attitude. No charges were brought against the policeman. Outside court, a senior police inspector said the undercover operation had been carefully planned and there had been constant liaison with the Legal Department.