ok Gareth,,, i really hope i`m wrong with this,,, i really do,,, i`d very much rather it simply be something like a zip tie on the pot joint boot clicking on the suframe or something,,, that would be much easier & simpler to correct hey?
But all the bearings inside the gearbox & diff are "swimming" in oil,,, & """IF""" it`s a broken bearing cage, then the shear weight of each ball falling & hitting the next ball makes a certain click noise,,, so,,, going on what you have described it just seems quite possible that`s what it may very well be
I wish i was there to actually listen to the noise myself
as i said, it is quite rare for a diff bearing cage to collapse,,, but that doesn`t mean to say that it can`t possibly happen,,, there are some cheaper "Plastic cage" type diff bearings out there these days so it could quite possibly be that too,,, it`s anyones guess as it`s very hard to diagnose a noise in someones mini via the internet forums
we`re all just trying to help,,, but i really do hope i`m wrong,,, either way you will certainly want to investigate it further yeah???