The knock for knock agreement no longer exists - it was an inter-insurer agreement whereby those subscribers agreed not to pursue each other for recovery of their outlay. This worked fine where both parties were comprehensively insured, but where an insurer had a predominately TPF&T or TPO book of business it became unfair (e.g, the TPF&T/TPO insurer would have no outlay even if their driver was at fault). The agreement was scrapped about 10 years ago: it had nothing to do with both drivers agreeing to settle between themselves. Pedantic, but factual. As previously answered though, there is nothing wrong with you settling yourselves, however, you will find that you are contractually bound to report the incident to your insurers.