Some posters have referred to the "Access to Neighbouring Land" legislation which gives the right to reasonable access for maintaining an "existing" structure. This means that if the extensions/walls existed when you moved in then neighbours would have the right to apply for permission to access your land in order to maintain them. However this does not mean that they can simply send work people onto your property without notice or arrangement. You have a right to require an up front appointment at a stated time/date. If workpeople are walking through your house and the task requires some effort on your part (being at home, admitting and supervising them, etc) then you have a right to treat that as "work".
Next time they ask make it clear that you regard it as a request to "rent part of your garden" to access as well as your work in arranging access etc. As these people are acting as your neighbour's agents you are entitled to bill them.
If the structures were built after you took up residence then they are "new" additions and the above legislation does not apply. In this case the NDN would have to negotiate a private treaty with you fort access to maintain the strusture.
You need legal advice which you can usually get free or for a fairly low fee for an initial consultation.
To think that London, or anywhere else for that matter, does not belong to any one demographic
A Good Year Film4 6.40 tonight
Recommendations please, for a stopover on the way to Loch Tay