If the leaves are going yellow or brown then by all means cut them down. You need to keep green leaves on for as long as possible as that is where the corms will get there energy for next years growth. If very untidy then do trim the leaves. Whenever we get a hard frost then they should die back pretty quickly.

I cut mine down 6 weeks after the flowers faded, which was ages ago now. If yours is the basic montbretia, the short, orange form, they probably finished later, but you can do it now.

Montbretia are tough little things and so won't mind being cut back now.

Hack away ... the basic montbretia wouldn't die if you nuked it!

:^)