As others have stated; the nude bits are Diane; but the sex bits are another inserted actress. Still a hot scene for Diane, & a clever edit that inspires even more jerking over Diane than the original of course. If only she'd done both parts....
The answer to the question, "Diane Keen or is it?", is yes and no: the softcore parts are Diane Keen; the hardcore parts are not. The film was shot as softcore but hardcore inserts were later shot with different actors. The softcore version was released in the UK; the hardcore, in the US.