JUST imagine. One moment you are relaxing with your feet up and a good book, and next you are interrupted by a telephone caller greeting you by name and wanting to know whether you would ''consider yourself busty''. That was the unnerving experience of our socialite friend Diane Butler who took just such a call from someone she described as ''a well-spoken gweilo'' who claimed to be representing Philip Wain, the beauty and fitness house. Explained Diane: ''After informing me that he was offering six months free membership for facials and massage he started asking me for my vital statistics.'' Diane, a kindly soul, pursued the conversation as best as she could but the mental alarm started buzzing when the caller's questioning took a let's-get-physical tone. ''Which is bigger, your hips or your bust?'' he inquired. That's when Diane brought into play a skill one usually gets to fine-tune on the cocktail circuit - i.e., name dropping. ''Please excuse me,'' snapped Diane at her caller. ''I'm quite sure Eddie and Deborah [the owners of Philip Wain], who are good friends of mine, would not be requiring you to ask such questions . . . '' The sound of the telephone being hurriedly replaced at the other end was the response to that inspired observation.