Canon refuses to send me the part, says the whole top cover will have to be placed, and I'll have to send the camera for it to be repaired under warranty. They don't have the "mode dial cap" listed as a separate part. I can get one on line for £19 and am tempted to buy one to save the hassle of posting the 5D.
That's a bummer. I'm not sure why you can get one online, is it from Hong Kong? Apparently the dial is permanently attached to the top cover, so if they don't stock the cap, I'd be tempted to buy one elsewhere as well.
Try one of the authorized repair places in the UK. If there is one close, you might be able to drive there.