Are you replacing gravel with gravel? If there is room to lay another inch or two depth without spill over onto the surrounding areas, I wouldn't bother with membrane. The extra depth of gravel should reduce seeds setting and any that do will be easier to pull out without having their roots growing into a membrane.
If you have weeds in the existing gravel it may be worth spraying with a dose of a residual weedkiller, such as Pathclear, before topping up the gravel.

I don't see why not

Thanks guys. I'm replacing it with slate, I was just worried if I didn't use the membrane the old gravel would start to work its way through.

You could spread a thin layer of sand and run a compactor over the gravel first.
If you have the time (ie the slate isn't arriving in the next day or two) you could advertise the gravel on freecycle - free to anyone who comes to dig it up and brings their own bags. I regularly see similar adverts on my local freecycle - and they seem to work.