This is the situation:

neighbour will not let me go anywhere near their land. I want to build
an extension onto the back of my house. One of the walls will need to
be adjacent to and with zero clearance to their already built
extension.

The Party Wall Act applies. You need to comply fully with it to ensure you
don't end up with major legal problems. You also need to ensure that
planning and building regulations are complied with. The first thing is to
go and talk to your local building control department. Their a advice is
free and it will give you an idea if what you want to do is feasible and
legal. Remember the neighbour is under no legal obligation to allow you on
his land. If you have to appoint party wall surveyors then you will have to
pay for the neighbour's surveyor as well as yours.