@Ajf350d Thanks for the link, good to know the service is there, if needed. As for my 'missing' part then it is in fact the axle joiner, the short grey tube.
@Peppermint_M I've not finished the build yet, I've other distractions. But as soon as I do I'll post a picture here.
It also seems that the general consensus is that it's very rare to get parts that need replacing, which is reassuring to know.