Sharing interent on mountain lion not working properly

Since upgrading to Mountain Lion I've been having trouble sharing my Internet connection. I've been able to use it since the latest patch but in the last few days its stopped. My devices can all see it, but when connected they don't have Internet. I've tried running a disk repair for permissions, as this was mentioned on a Lion blog, but this didn't do it. I've tried re-creating the share, but that didn't work either. All devices have been reset and I've tried renewing the lease but nothing seems to work.

I know it has worked, so it should be possible to fix it, but I'm out of ideas now.

I've got a late 2010 Macbook Air, with the latest updates applied. I'm using a USB 3G dongle and that connection has been set up to share internet. The wifi logo changes with the upwards arrow and it does appear to broadbast my wifi, but no interent is available though this. The internet on the Mac works nice and quickly so this is not the issue. Like I said, this has worked before, so I know all of this should work, but I'm not sure what's broken it.

Any help would be much appreciated.

Thanks