How did I completely miss 5 threads of mumsnet gold till just now.
Caught up just reading OP posts only. Is "put up a fence" this thread's "cancel the cheque"? If you can afford it I would instead suggest a good solid garden bench like this (cheaper ones down to £40 or so also available but make sure it's not too lightweight)
I would suggest writing a formal letter along the lines of:
Dear CFN
Given the complexity of this situation I hope that you agree that it is best to keep all communication between us on this matter in writing.
For the avoidance of doubt I must start my confirming that you do not and never have had my permission to access my land in any way whatsoever. The work that has been done illegally to your flat includes a serious trespass onto my land which is unacceptable.
I understand from the council planning department that you will be required to revert the building structure to its original state at your own expense. Obviously this will require further access onto my land. Please ensure that I am given at least 2 weeks notice of any such work in writing. I will of course require appropriate compensation for the intrusion and inconvenience, and my garden must be completely tidied and cleaned of all mess and leftover building materials when the work is complete.
I would prefer not to discuss this in person as we both have the right to enjoy our private property in peace without unpleasant scenes, so please do reply in writing to let me know your plans to set this unfortunate situation right as soon as possible.
You will note that I have placed a bench against the doors in order to protect the privacy of my land. Of course in the event of notice being given for any pre-arranged work, I will agree for it to be moved. Any other access into my land except for rectification work for which due notice has been given shall be trespass and shall be prosecuted.
Yours faithfully
WhatIsThisWatt