I know this sounds a bit cheap, but superglue/adhesive is too brittle to make a good fix. Especially seeing as the unit cracked unevenly.
I took the unit off the arm of the pushchair by pushing the little metal clip in at the bottom and pulling it down.
The rest of the unit which was still inside the wheel needs a pair of tweak nose pliars and a flat head screwdriver (and a lot of patience). You need to use the pliars to grab the unit in the middle of the little circle, then use the flat head screwdriver to push in once side of the expanding clip in the middle.
Once you have both broken parts, hold them together in the correct place and wrap A LOT of cellotape around the two bits. Then clup both bits back on and hey presto, it works!!
Just taking it for a "road test" so to speak now, but it was an easy fix. You just need to be careful when removing the parts that you don't break them beyond recognition!
I tried to pull the two parts apart with my hands after and to be fair, the fix is probably stronger than the original flimsy bit of plastic.
If anyone wants any help on how to fix it (pics or more of a descriptoin of how to get this bits off) drop me an email at [email protected]