It is a spoof sketch which was shown at the start of the show in which David 'phones' Ewan McGregor for advice.
The Last Leg (which was known during its first series as The Last Leg With Adam Hills) originally ran alongside the 2012 Summer Paralympics every night following the main coverage on Channel 4. It gave an alternative review of the day's events and featured a mixture of comedy, guests and Paralympics highlights.
It has since become a weekly show and is co-hosted by Josh Widdecombe and Alex Brooker. The Last Leg is summed up by main presenter Adam Hills as "Three guys with four legs talking about the week." This is based upon the fact that Hills was born without a right foot and Brooker had his right leg amputated when he was a baby.
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The Last Leg section of DavidTennantOnTwitter.com