You have been far too trusting and co operative with Morris Homes and now they are taking advantage.
I would have demanded £50 a day for the scaffold on the drive at the outset, you let them have it for free.
The first thing to do right away, is to refuse access over your property ie the driveway.
Write to Morris and say in no uncertain terms, they are NOT TO TRESPASS on your property/driveway.
If they ignore this, state that you will require damages in the region of £1,500 per instance.
If they refuse to pay, tell them the matter will go to court, with all the publicity you can muster!
You should also inform them in the letter that they are responsible for damage to your property (the grass) and that this must be made good within the next seven days, you can always rip it all out and lay your paving the moment the weather is better!
Any gaps in fencing should be filled too, especially if it is your fence!
And yes, why not put a sign on your house warning others "Don't buy a house here without talking to us!" too.
And you could talk potential buyers outside the sales office at the weekend. Or put fliers on their cars!