In his hands is a board bearing a message of congratulations with a toy duck stuck to it.
Nicknamed 'the Union Jack man' by the Queen because of his fondness for wearing national colours, he has been a fan of the family ever since meeting her mother, the Queen Mother during the Blitz.
In July he spent 12 days awaiting the birth of the royal baby outside the hospital in Paddington.
On Wednesday Terry, 78 was joined by other royal supporters with cakes, balloons and pictures of the Windsors.
Shortly before the service at 3pm, well wishers will be rewarded by the sight of the Duke and Duchess of Cambridge arriving – it's not known if they will get out and greet the crowds.
Other senior royals including the Queen, Prince Philip, Prince Charles and the Duchess of Cornwall are also expected.
Of course, members of the Middleton family – Carole, Michael and Kate's siblings Pippa and James will play a part of the joyous occasion too.
As godparents for their child, new parents William and Kate have chosen six friends and one family member – William's cousin Zara Phillips.