Legardere then brought Castres, down to 14 men with Steve Kefu yellow-carded, back to 15-15 with two well-struck penalties.
Australian centre Kefu, 29, arrives from Castres Olympique, number eight Ward-Smith, 31, from Bristol and 23-year-old wing Varndell from Leicester.
Steve Kefu sent in David Lemi after the break, while the Dragons lost Nigel Hall, Phil Price and Tovey to injury.
Yet they took the lead for the first time on 46 minutes as Kefu's break sent Lemi in for an unconverted try.
Kefu's rugby future was in doubt earlier this week after he took a banned substance prednisone to treat viral disorder Bell's Palsy.
Then, within five minutes of the restart, centre Steve Kefu and full-back Mark van Gisbergen combined well to send Lemi over the line.
Dan Biggar had punished some French indiscipline to put the hosts in front but Wilkinson fed centre Mafi Kefu at the base of the scrum.
But tries from Steve Kefu and Tom Varndell, and two Walder penalties, won it for Wasps, with Paulica Ion's late effort for the Exiles mere consolation.
Wasps centre Steve Kefu had just fired a kick directly into touch when Robson became embroiled in an off-the-ball scuffle and aimed a head-butt at Wasps scrum-half Joe Simpson.
The Exiles squandered two chances to score on the Wasps line and the hosts made them pay when Kefu scored from close range to cap an attack that started with a searing break from Jacobs.
Wallaby centre Steve Kefu shrugged off illness and the threat of a drugs ban to score two tries in the opening period for the Reds before the Chiefs levelled at 18-18 midway through the second half.
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