I'm just in the middle of reading 'A Murder Is Announced', and it appears a very strong allusion to the spinsters Murgatroyd and Hinchcliffe being lesbians. An early chapter depicts them in somewhat stereotypical garments (Hinchcliffe in corduroy slacks and battledress tunic.... 'she turned her head with its short man-like crop and weather-beaten countenancer toward her friend... Murgatroyd 'wore a checked tweed skirt and shapeless pullover of brilliant royal blue.") And a classic oneliner from one of the women ('men are dirty dogs'), together with them living together at Boulders all seems to point toward it.