Last night I realised that I had an unused Brooks Flyer still in its box (the Flyer is a essentially a B17 with springs - in between a B17 and a B67). I replaced the Phenom Expert on my fattie with the Brooks Flyer so will see how it works out.
The Brooks Flyer is a really heavy saddle, but in the context of a fat bike, my own weight and my leisurely rides, it's not going to be an issue.
I did (finally) remove the spoke reflectors on the bike too, just to save a few grams.