Any question answered...

I've just had a quick ring round- they are shut, or not answering the phone anyway.

The iBox comes with three reducing boss' and one blanking plug, for (presumably) the most common setup which is one shower outlet.

I need two outlets, so I need to swap the blanking plug for a reducing boss.

Hansgrohe will sell me one direct, for £13, and it'll take a week to get it- I don't mind paying that (although it seems steep) but I'd like to be able to go and get it today.

£13? A bargain!
Hansgrohe take the piss -after you've bought an iBox, valve, and finishing set for >£350...
What you have there is a dead common 3/4" to 1/2" male reducer. Any plumbers merchant will have them - If I'd seen this earlier you could have had one of probably 1/2 a dozen I've got in my plumbing box, pick up from my arch in Bermondsey.
If all your pipe work is 15mm you could just use a 3/4" (male) to 15mm compression or solder fitting and do away with the reducer bushing.

Have fun!