...most likely a stupid Q by a stupid person

Posting a quick update and thanks as always to hoolio - I should have read your first post better. While I had added the internal CA cert of the CA used to issue the client certs to the CA bundle on the F5, to get the client auth to work I had to switch the trust CA's to the CA bundle (I incorrectly presumed this was the default setting used on the F5 so would not need specifiying but I guess I was wrong). with regards to firefox, while the internal CA cert was installed I didn't have a client cert installed (IE prompts with an empty box...)