ELEVEN United Democrat legislators held an overnight sit-in outside Government House last night to protest against Governor Chris Patten's alleged failure to combat property speculation. The liberal legislators, led by its chairman Martin Lee Chu-ming, vice-chairman Yeung Sum and veteran Szeto Wah, said after a one-hour meeting at Government House yesterday that they were disappointed by Mr Patten's failure to offer any concrete measuresto stop property prices rising further. ''We fail to see a prompt and active response from Mr Patten,'' said legislator Lee Wing-tat. Mr Lee said although Mr Patten admitted that the issue was of paramount importance, he did not say how property speculation could be cooled. He said the party would hold an overnight sit-in outside Government House every Saturday until the problem was solved. In a letter given to Mr Patten yesterday, the party warned that booming property prices were a ''time-bomb'' which could disrupt social order if the Government turned a blind eye to the problem.