Invoicing rent waives the breach of failing to notify you of the assignment.
What I suggest that you write " without prejudice" to the new owner proving your identity as freeholder ( HMLR office copies) and explaining that
1 When they purchased they were aware of ground rent being due
2 That their solicitor would have asked the seller about any outstanding rent and should have seen confirmation of payment.
3 That if rent was outstanding when they purchased they took on that liability.
While the 6 year limit is in place, the loss of rent before that could be the measure of damages from the failure to serve the notice. Explaining that, offer that if all arrears are to be paid up to date you will waive any further action, and send demands* for them to pay.
In practice the legal costs would wipe out any rent due, so this is an attempt to get rent in, and if they refuse then yes prior to forfeiture proceedings you can ask the mortgagee to protect their interest.
* Demands must contain the name of the landlord and address in England & Wales for service inculding proceedings of tehlandlord and contain a notice re section 166-http://www.lease-advice.org/documents/Form_of_Rent_Demand_Notice.pdf
Based on the information posted, I offer my thoughts.Any action you then take is your liability. While commending individual effort, there is no substitute for a thorough review of documents and facts by paid for professional advisers. More ramblings atleaseholdpropertymanager.blogspot.com