In reply to Somerset swede basher:
This happened to me (gearbox) when I went over to pick my brother and all his worldly possessions up from Grenoble. On the way back, the gearbox packed in, got rescued off motorway, found a hotel, garage gave me the bad news next day, so I hired a car, drove it to UK (with all my brother's stuff), drove it back a few days later to pick up my car. Got back to the UK just in time to go to the Paris airshow with some friends (I didn't drive!).
So, I crossed the channel 6 times in about 7 days in 3 different cars. Right hand drive car in UK driving on left, then driving on right in France, then left hand drive hire car driving on right in France, then driving on left in UK, then back to France driving on right, pick up my left hand drive car, drive on right, then drive on left back in UK. Then sit in back seat whilst someone else drives me back to France.
Most awkward was driving French car in UK.
Amongst all that I spent about 60 hours in a row awake.
It wasn't really worth it, car was cr*p but I loved it and couldn't leave it out there.
Good times