P.J. Denyer
Penultimate Amazing
Assuming I've not cocked-up the link, here is Robin's 'letter' on his Instagram page.
A question that comes to mind is, what will Brian Cox do? Carry on as normal with a new partner (Dara O'Briain?) or resign as well? The latter would cause problems with his other Beeb programmes.
I don't think it necessarily would, if Cox were to say that he just feels it was a partnership and so it's time from his point of view to bring it to an end rather than start afresh with a new co-host I suspect the BBC would happily let it drop (and if they choose resume with all new hosts as they did with Top Gear). If he involves him self in the reasons for Robin's departure then it's likely to become more awkward.