WHEN First Pacific Davies taipan David Davies took 40 of his close pals over to the Bela Vista in Macau for a belated bash to celebrate his 50th birthday he planned to spring a surprise - by announcing his engagement to interior designer Alison Henry. But if there was a slight hint of trepidation as he broke the news, it was understandable. For despite making frantic calls all day to his 79-year-old mother, May, back in England, Davies was unable to make contact with her and seek her blessing. ''Everyone I called kept telling me she had gone to Brighton for the day,'' he complained. Imagine then his disbelief when at the opportune moment, who should enter the room but his mother, who had flown in secretly, expecting to be joining just a birthday celebration. No date has been set as yet for the nuptials between Davies and his New Zealander fiancee. But, Davies told me: ''My fung shui man insists that I must get married before the year is out.''