I used copper washers as an emergency repair for the old V6 as mentioned above - they lasted for a year until the car was sold. They came in one of the copper washer multi-packs from Halfords and I did flame anneal them as Jim recommended.
I would not recommend copper long term though due to the risk of electrolytic corrosion between the aluminium alloy and the copper! There was a little bit of corrosion starting to show around the copper before the car was sold but nothing major.
On my second V6 when the Banjor started leaking I fitted the OEM aluminium alloy washers - still available from Citroen. Here's some pictures for you:
New and old washers:
After fitting:
If I had to give any advice it would be to be very careful not to get any dirt or muck inside the pipe work or housing when removing it - clean thoroughly around the area first as any dirt getting into the oil in the gearbox can cause major problems. Also when refitting the bolt don't be tempted to over-tighten - it doesn't need to be super tight and as the banjo bolt is mostly hollow it looks like it would be fairly easy to twist and snap it! I would just nip it up with a ratchet and no more. If the faces where the washers go are clean its not going to leak again in the remaining life time of the car.
A pretty easy job really and plenty of access with the wheel off.
If the leak has been there for many miles then the gearbox oil level may be low - so ideally you should check the oil level, but the method of doing that is a whole other story.
