A SNATCH thief who knocked a 66-year-old grandmother to the ground with an umbrella was jailed for 21/2 years yesterday. Wong Chi-keung, 26, a former casual lighting worker with ATV, was caught red-handed by an off-duty policeman as he fled from the scene of the late-night attack. The victim, Cheung Mai-chee, was forced to spend two days in hospital recovering from the blow to her head. In the District Court yesterday, Wong, a drug addict, admitted wounding and robbing Mrs Cheung on February 5 this year. Wong pounced on his victim as she walked by a park on Chuk Yuen Estate with her granddaughter, Lam Sze-ting, prosecutor Sondra Riley said. She was struck over the head with an umbrella Wong had picked up minutes before. As Mrs Cheung fell to the ground dazed, Wong fled with her handbag. He was apprehended by the off-duty officer who happened to be passing by. The victim's granddaughter, Ms Lam, later picked out Wong at an identity parade. Defence solicitor David Ravenscroft said Wong attacked Mrs Cheung on the spur of the moment because his drug problem had got him into debt. Judge Gould said the 21/2-year jail term would run consecutively to a 10-month sentence Wong was already serving for another offence.