Down to 70 mpg on my 48 mile commute, either it's the cooler weather or me being a bit more eager with the throttle

What mileage are you at? what's your air filter looking like dirt wise? Mine was pretty dirty at 4k, nothing a hoovering didn't mostly fix. But they do tend to dirty up quite quickly and in my experience, often well before when Suzuki recommends replacement. If you check it, do so with the bike on it's side stand. It makes getting the filter in and out easier and the rear suspension compresses slightly and lowers the cylinder head/engine unit down just a little giving more access space to lift the lid on the airbox.

ps. I appreciate you may already know this stuff. Just asking/suggesting

That's OK for a 400 I would have thought

I'd take that...
My Xmax400 is averaging 65 mpg on a stop / start commute... streatching to 70 mpg on a run...
The old XMax250 was returning 80 mpg...

Yes, 70mpg is not bad I'm guessing, but don't know mj's riding style, road conditions encountered or state of bike. But these later 400 bikes are incredibly good on fuel, much more so than earlier bikes. I get more, but my commute doesn't take in much traffic. If I go to the office it's a 26 mile one way run at the moment on the A12 and she does around 76-78mpg at peak traffic times. Even if I go through a couple of towns on the way it's never less than 75mpg even in the cooler weather. I don't flog it, but don't hang around either.