For anyone in future who needs to do this, here is what is involved. Citroen see the gearbox as a "lubed for life" component so they haven't made access a major feature.
You'll need an 8mm square oil bolt tool, Halfords do one for £4.99. Everything else is normal bolts so a socket set will do.
Jack the car up and place axle stands under it as you'll be needing to get underneath. The bolt you are looking for is has an 8mm square hole although on mine, the tool didn't seem to fit but it only needed a light tap with a hammer to convince it. It's situated at the front of gearbox near the line where it joins the engine. Following the idea of sealed for life, access to the bolt is obstructed by a bloody great steel bar that goes from one side of the car to the other and is held in place by 4 x M10 Bolts. You only need to remove the bolts on the gearbox side, loosening the pair on the other side allows the bar to drop down far enough to be out of the way. On mine (2012 model) they were pretty stiff because of rust so plenty of WD40 and a degree of judgement are a good idea. If they break, it's a beggar of a place to drill so take your time.
Remove the drain bolt and allow the oil to drain into a bowl while you get to work under the bonnet.
You need to remove the air hose that goes between the injection system and the airbox that it connects to on the front of the car. Switch the ignition off and give it a few minutes for the computer to do what it needs to do shut down cleanly.
Remove the positive lead on the battery and tuck it out of the way. Remove the wiring box from the top of the battery, (slides away from you and fold it over to the right. Lift the narrow plastic cover from the ECU housing to the right of the battery and pull the ECU upwards. It will fold down in front of the battery. Lift the cover from the battery housing and remove the negative lead which can be tucked away at the back. To the right of the battery, in the space under where the ECU was fitted, there is a single 10mm headed bolt with a pressed steel plate under it. Remove the bolt and the plate, the battery lifts out.
There are several cables that are tied to the front of the battery tray. Prise these out with pliers, and remove the two bolts in the bottom of the tray that hold it down. The tray will lift at the front and you can pull it out. There's also a small plastic fitting that sets the size of the battery in the tray. Make sure to note which set of holes it goes in and put it somewhere safe.
Under the battery tray is a metal bracket. Removing this not essential but does ease access and as it's held in by one bolt, it's not too much of a pain to remove. Remove the bolt and the plate slides back and lifts out . On top if the gearbox to the left is the gearbox breather, a round plastic cap around 30mm diameter on top. This just pulls off.
Put the drain bolt in with a fresh copper washer and tighten to 40Nm. The bolts that hold the steel bar in place can be reinstalled, with new ones if you feel it necessary. They're 10.9 grade hardened so if you buy new ones, make sure you get the right grade. Mine were pretty rusty but 10 seconds with a wire brush in a Dremel cleaned them up well. Standard ISO torque setting for this grade at M10 is 75Nm so this is how tight I made them.
Get a bit of 1/2" external diameter vinyl tubing and a funnel that you can push it onto. Stick the pipe into the breather and pour in 2 litres of 75w/80. Refit the breather and reassemble in reverse order.
To be honest, It was less of a pain that I was expecting. The positioning of the bar on the bottom of the car was a fun surprise the rest was pretty straight forward.
Anyway, hope this helps anyone in the future.