UK Can't get stopcock 100% closed to allow a tap repair

[DIY disaster]

I have a plumber coming tomorrow to change a cartridge on a badly dripping tap on my bathroom mixer unit. There is no separate isolating valve for this tap so I'll have to close the stopcock. I tried closing it today but it won't go absolutely 100% closed and there is still a very slight flow from the tap. Will the cartridge still be able to be changed under those circumstances?

 

[Chuck]

Won't be a problem for a pro plumber. Get them to advise whether to service / replace / add full bore isolator in series / etc. the stopcock while they're on site.