I've just sold a 207 1.6 HDi and I changed the gearbox oil just before I sold it. The manual gearbox oil filler is below the battery tray and so I found it necessary to remove the battery and the battery tray to gain access. Once the battery tray was removed access was quite easy. It was a black plastic screw in cap with a little air vent on the top. You need a socket set to remove the filler cap.
I had a bit of a job trying to find out the gearbox oil quantity with some people saying 1.9 litres and some saying 2 litres. I drained the gearbox oil (it was quite dirty at 65K miles !) and filled it with 2 litres of new gearbox oil. There is no way to check the gearbox oil quantity other than draining it, measuring out the quantity and then refilling it.
Oh nearly forgot......on my 2011 model after disconnecting the battery I had to unclip the engine ECU from the side of the battery cover and place it on one side out of the way. I didn't actually disconnect the ECU connectors.