I think I'd write a brief note stating that your permission should
have been obtained before these pipes were installed, that you have
not given your permission and that you would expect all further work
on the installation to be stopped until you have received written
details of the works for consideration. When you know what it all is,
only then could you consider giving permission.

Suggestions:
1) Keep note brief and to the point. i.e making no other points that could
weaken your argument.
2) Try to obtain some form of agreement (or a nominal payment?). That will
then place on record that it IS an intrusion onto your property.