If you know anything about the cost of second-hand cars in France, you won't be surprised to read that I got our new-used car in England. If you're thinking of doing the same, you might find the following useful.
The first thing to do was switch the headlights over to left-hand drive headlights. We used Headlampshop on ebay, following a recommendation, which all went smoothly (and cost about €260). Because the fitting wasn't a simple spanner and hammer job, I used our local garage (which cost about €80).
Then I needed a Certificat de Conformité from Toyota France. This is a piece of paper that says the car can be driven on French roads. I tried to get around it, but discovered it is currently compulsory. A mugging at pen-point, if you like - or even if you don't (which cost €135 but is dependent on car make, type, power etc).
Armed with this, I could put the car through its Contrôle Technique. This is the French equivalent of the UK's MOT but only needs to be done every two years. The mechanics are reassuringly thorough and the whole thing takes about 30 mins (and cost €68).
Armed with this, I could go to the Sous Préfecture in Bergerac. I also took the UK registration document, Certificat de Conformité, my passport, a justificatif de domicile (recent bill to show we live here), my insurance documents and anything else I could think that I might be asked for. The woman handed me a Carte Grise (vehicle registration) request form and wrote on the bottom that I was missing a "quitus fiscal". This I needed to get from the Hotel des Impôts. I went and watched a woman complete, by hand, a beautiful form - in triplicate - who handed me the top copy. It seemed pointless, although charmingly antiquated, and I began to feel empathy towards people in offices doing this for a living (or more likely, a retirement, but that's a different story). I went back to the Sous Préfecture and handed all my papers over again, answered a few questions, watched the woman trying to make sense of forms she saw every day (with increasing empathy), was given a new form to take to the next window with everything else and paid for our new number plate (cost €96.50 and is also dependent on the type of car).
I knew from before that I had only a short time in which to change the plates or face a fine, so I immediately went back to the garage. They ordered and fitted the plates for me (for about €50).
Now all that remains is to go back to the Contrôle Technique and have them put a sticker on my shiny new Carte Grise, and to tell my insurance company about the change, so they can print out a new - and final - piece of paper for me. I don't think this will cost anything.