No u don't need a Transit Visa!!
http://www.flyslm.com/visa.html
And I don't have first-hand experience, but don't waste any time go as
quickly as possible....The guide says it takes 20 mins by Express...
http://www.heathrowairport.com/heathrow-airport-guide/travel-between-terminals
http://www.heathrowairport.com/heathrow-airport-guide/flight-connections/terminal-4-international-connecting-to-terminal-2-international
Also, you can read one traveler's experience of shuttling here (in between gates at T5 though
)
http://www.canadavisa.com/canada-immigration-discussion-board/landing-in-canada-all-you-need-to-know-t36735.0.html;msg4351921#msg4351921