What a nightmare.
I'm a solicitor (property) and I am saddened that a solicitor's letter wasn't sent to them from the off making clear to them the easements in force at the end of the drive.
There will be a nuisance covenant in the title also, to all your properties.
You mentioned a management company above - this suggests to me that it is a newish built development with the common areas maintained and owned by the management co.
You could have contacted them or the beneficiary of the covenants in your title (builder usually) and asked them to impose the nuisance restrictive covenant.
Their title deeds would have been easily accessible via land registry for £6 (register and plan).
Your solicitor then would have all she needed to highlight who can go where.
What has happened here is that you, knowing your rights, have been passing over what they believe to be their land without a care in the world. They have become incandescent with rage at this perceived intentional sleight and it's grown and grown and lead to the cluster fuck of issues you have now.
Am glad it appears to be sorted.
I fucking hate this shit when clients come to me with them. I'm aggressive and meet fire with fire - the only way is to nip this shit in the bud.
It's a battle of wits and the other party is unarmed.