From TARDIS Index File, the free Doctor Who reference.

Diane Holmes
Also known as:	{{{alias}}}
Race:	Human
Home Planet:	Earth
Home Era:	1953
Appearances:	TW: Out of Time TW: End of Days
Actor:	Louise Delamere

In 1953 Diane Holmes was a pilot. On December 18th she, John Ellis and Emma-Louise Cowell were on their way to Dublin. But somehow the plane (Sky Gypsy) flew through the Cardiff Rift and ended up in present day Cardiff.

Diane and Owen had a relationship, but she decided to fly away and try to find the Rift again, to get back to 1953. Whether she succeeded, or if she was killed or otherwise lost in the attempt, remains unknown. (TW: Out of Time)

Later, an apparation claiming to be Diane appeared to Owen, but it is not known if this was really Diane. (TW: End of Days)

Diane was in flashbacks in TW: Combat, Captain Jack Harkness & A Day in the Death