A London bus, but not a Routemaster. There were very few Routemasters left in service by then, It was most likely the Dennis Trident 2 (http://en.wikipedia.org/wiki/Dennis_Trident_2, http://www.google.co.uk/imgres?imgu...a=X&ei=bnvXT4ynDIr10gGF78iUAw&ved=0CBIQ9QEwAA) which was on route 30.
You are correct about the bus type, a Dennis Trident 2 (fleet number 17758, registration plate LX03 BUF, two years in service at the time) operated by Stagecoach London. See:
http://uk.ask.com/wiki/7_July_2005_London_bombings#Tavistock_Square_bus