@stephenw10 Hi Stephen,
Just to say after so much trouble with this, I learnt that a work colleague of mine had the FS BOX which I just borrowed to recode the SFPs to Intel. As soon as I had done that and plugged them in, they came to life immediately without any need for tunables or other tweaks to get the link working. It's working on autoselect / autodetect with a fresh install of pfSense.
Thank you so much for helping me out with this, but I hope this thread is of use to others: any recent FreeBSD install or security appliance designed on top of FreeBSD using Intel SFPs, you really need Intel-coded SFPs otherwise it just won't work!