Ann had two sisters - Helen Kay Stephens (1934-2024), who married for the first time in 1967 and Angela Kathleen Stephens (1928-1982), who married in 1979. Their mother's married name was Kathleen Maud Stephens (1904-1995).
Anyway, the reason I summarised all of that is because Helen, Angela and Kathleen seem to have all been living at the same address from around 1960 to 1965 - 8 Chepstow Mansions in Kensington (Bayswater?). I've attached one electoral register from 1964.
I haven't seen Ann Stephens hereself in any of these electoral registers, but there also doesn't seem to be any evidence of her working as an actress after 1959 or so, so I was wondering if that's the time period we should be concentrating on - 1959 to 1966. At the moment we don't even know what country she was in, let alone whether or not she was married!
One other minor point - the death of Ann's sister's mother-in-law, Mollie Dorr Sledge, can be found in various sources on ancestry, and they almost all say that she died in London on the 13th May, 1965. In fact it seems that she died in Addenbrooke's Hospital in Cambridge. I mention this because if Ann is reported dying in a car crash in London, it might not have been in London. US newspapers probably would have put London for anywhere in the UK that wasn't Scotland.