Finally fitted the new idlers last night.
The package included two, one replacement foremost return idler, one replacement power side idler. Both look meatier than the units they replace, both allow lateral float along the spindle.
For the front one, this is maybe 5mm.
The power idler allows 10mm of travel, and loses the little 'ears' from the chain guide. This is welcomed, as I was frequently catching them and bending the guide out of shape. Unfortunately, the new idler seems incompatible with the Furai, as the thicker tubing required for the hinged rear end causes the idler to foul the underside of the seat. I'm waiting for a response back from Challenge, who to be fair, are being very good about the whole thing.
Suspect I'd get further with pics than words, so see below for the powerside issue