Having added @jsyjr's firewall rule and left things running for a few days, once again I had some interesting behaviour. Firstly - unfortunately this did not fix the original kernel warning issue, I actually received more than double the warning logs over a two day period (+200 per day) but I don't notice any performance degradation at all. I can't see any particular patterns over timings when the warnings are logged either - seems pretty random to the untrained eye.
The firewall rule did, however, fix another long standing irritation of mine - I have an incredibly chatty Dnsmasq which fills up my logs more than the kernel warnings. I have set option quietdhcp '1' so I only receive SIGHUP request logs from clients....... but I do get loads of them (dnsmasq re-reads /etc files every 10-15 minutes on average). With the firewall rule in place - my log files were much quieter and the only time dnsmasq re-read /etc files was on start up which is good.
So at least I now know that it's an IPv6 enabled client that is generating the SIGHUPs which will help me narrow down the search somewhat. Obviously, if anyone else has any suggestions please do put me out of my misery!!
I've now disabled the firewall rule but will keep it for on-going testing purposes so thanks for that. So I guess I will just have to systematically disconnect each device and work out what hardware is generating the warnings and SIGHUP requests. I'll have to find an appropriate time to do this over the coming week....... and will report back again.