What are the documents required for extensions of Visitor Visa. Parents came for their grand baby's birth.

Also ,UK consulate website says:
"You will be exempt from the DATV requirement and may be able to transit without visa if you hold one of the following.
A valid airline ticket for travel through the UK as part of a journey from Australia, Canada, New Zealand or the United States of America to another country or territory, as long as you do not transit (pass through) the UK on a date more than six months after the date on which you last entered Australia, Canada, New Zealand or the United States of America with a valid visa for that country."

Does this mean, a transit visa is required - even if, my parents have 5 yrs multiple entry visa stamped on passport which expires in 2012. They plan to stay beyond 6 months they were granted at POE. Will they have any problem while back via London?? 

Seems like London Transit is a pain.

Please suggest your experiences. Urgently required. Thanks in advance.