[VinnyP]

I don't think the 400D has a sync cable socket. You will either need a hot shoe adaptor Cheapest at well under £10 or a wireless solution. Pocket wizard is the best known but is not cheap. A lot of people who know about these things swear by budget versions from E-bay this is a lot easier as it reduces all the cables running around. For a basic version it's around £25 delivered. If you use slaves for the other heads then that's all you need.