To be fair Tambo the original was Jimmy Young but I was going with the fact, the OP said it sounded like a girl group, all good versions in their own right though.
I agree with missprim it certainly sounds like the Fleetwoods version. Regarding the original version Tamborine... I first heard it sung by an excellent jazz/ballad singer called Al Hibbler: He had a big hit with it in 1955 ( giving my age away here) but as you say many people have recorded it so who knows?