Hi Dirk,
Yep, I'm in Chertsey.
If the client is making it to the VIP that you have the rule configured for, there should be logging for the HTTP_REQUEST event (regardless of whether the client is presenting the cert or not).
Are you testing by not sending a cert at all, or are you testing by sending a cert initially and then not sending a cert?
Is the connection reset or is passed to the portal-apps pool?
If you aren't seeing any logging whatsoever for requests where clients don't present certs, you might try restarting TMM (bigstart restart tmm) and retesting.
Aaron