I was warned that it wouldn't be possible to fit the spare onto the Bongo hanger (because the centre hole is smaller than a standard steel) so I got an old one and had a look at it.
Turns out that the only thing you need to do is to cut off the noggin on the wire which prevents the holding plate sliding up on the wire.
After sliding the plate up the wire it's possible to first thread the spring and end plate through the wheel centre and then the holding plate. End of problem.
Clamping noggin on the wire removed

Then got the Bongo up on the ramps to fit the modified hanger assembly only to find that on our 2001 model the hanger is different and will take the the alloy without any modding.
Later type hanger. A lot thinner.

Goes straight though the wheel

Only slight mod needed was to wallop the rear supporting bracket back a bit. probably wouldn't have had to do this if there was no tow bar.

All fits nice and snug.