Just as an addendum to this.
My van is a Swamper, shod with Wolfrace Outland wheels on 235/55 18 AT boots.
I've moved the spare from underside to tailgate, partly for the look and partly because if I scored a puncture in an inconvenient place it could be dangerous to be fannying about underneath the van trying to retrieve it.
I stuck with the steel 16 incher as a spare - a wheel to match the alloys on its own is £300 plus tyre, and frankly I'm not that bothered anyway.
So I had the spare shod with a 205/75 16 all weather boot, which is within 0.3% the same diameter as the boots on the van so it's perfectly useable as a "space saver" on either axle.
Just to keep things looking tidy I had the wheel power coated hammered black, which was only £40 as I was happy to wait until the coaters had enough work from other customers to do a full oven in one go. It sort of looks halfway between gloss and hammered finish, but is clean and tidy and that's my main concern.