Although it is not known to all and sundry whether Mark is indeed selling down his shareholding, his position falls under 'Restricted Persons" definition within their policy 'Trading in Company Securities'.
There appears to be no specific restriction on departed employees or Directors - except - where they "remain in possession of Material Information".
I would think that Mark certainly does "remain in possession of Material Information", in which case technically he would be restricted from selling under the policy, though for how long is also unknown.
Notwithstanding other Insider Trading laws, it is possible that Mark 'applied' to the Board / CEO via the 'Request for consent to Trade in Relevant Securities' and was deemed Ok to sell his shareholding, despite possessing 'Material Information'.
So now that's clear, it's all still very unclear, whether it's Mark dropping in the 4-500k shares trades (two today so far). If it was though, at that rate he would be gone from the register fairly soon.