Any question answered...

£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!

Half the battle (if not more) is finding out what the common reference term is for these items- tricky to search for something that you don't know the name of.

Armed with "3/4 inch to 1/2 inch male reducer" I found one for a couple of quid in a Leyland SDM round the corner, however it is too short, annoyingly.