The reason you won't have recieved a response yet is because all the lecturers will currently fall into one of two camps; 1. Busy preparing for the new term and new intake. 2. On holiday so they don't get caught up in all the work preparing for the new term and new intake.
It is certainly worth e-mailing them to ask about this. I imagine that kind of experience would be a big plus and in the admissions tutor's eyes may compensate for the lower grades, however sometimes they have concrete cut off points built into the computerised part of the admissions system and if you don't meet them your application can get rejected before its even seen by a tutor. If you have other points like this which compensate then they can go delving into the system to retrieve your application and make sure it is considered.
As you've already e-mailed admissions this is a pretty moot point. If they do have these concrete cut offs (they still don't apply to all courses and aren't always A-level conditions) they should take a note of your name as they read it and make sure this doesn't become an issue. Either way, don't panic too much about it all, go and enjoy the open day and ask as many questions as you can possibly think of while you're there, if you haven't had a reply by then, you can always ask someone from admissions in person.