Neighbour seems using parents (semi detached ) houses garden to transport builders and goods

Worried they are gaslighting them claiming that they deserve right of passage into their garden. My parents allowed them for fire emergencies other than that they ask my parents to open to door. The door into the garden and the fence separating our houses belongs to us.

Lately they have been using the garden to transport builders goods etc, , dumping wood there (though they clean up after)when my father confronted them they claimed because my mother made an extension there 11 years ago that they blocked access into our garden (when there is another gate thatallows them to get into our garden ). They said they want to do the same on Saturday despite my father asking not to hog the garden like this

Ive checked the online documents , title register and title plan off thr gov website and it doesnt say thay they are allowed to use the garden like this . Any ideas on what i should do to check legality other than this or if anyone knows?