I think it would be nice if they had acknowledged your email by now Phil, even if they are too busy. They could say they have received it and are processing it or something to that effect.
It's like waiting in a shop for service. I hate being ignored. When a shop is busy it's so nice when they see you and say "I'll be right with you" rather than leaving you waiting with no acknowledgement. It makes all the difference.
Sending an email is a bit like waiting in a shop I think.
Cheers
Gary