I thought I'd share how I've gotten on. There's a 2.1a charger that maplins sell that I thought might do it, but alas no - still comes up as "Charging(USB)". I know taFFer solved the problem with a little solder, and a lot of forum threads also suggest the same fix, but the last time I picked up a soldering iron actually ended with a trip to A&E (long, embarrassing, story) so I was still keen to find an off the shelf solution.
Anyway, after reading a lot more forums, I found a few different people recommending the same motorola charger. Amazon UK sell it, so I stuck in an order. Details below:
MOTOROLA P513 MICRO USB IN CAR CHARGER, £5.45*
*above item(s) sold by and shipped from Keep Talking
It arrived today and sure enough simply plugging it into the car results in it appearing as Charging(AC). I've not gone on a long enough drive to know about charging speed when gps/bluetooth/etc is all going, but if anyone else is after a cheap car charger that just works without hacking your own cable I think this is the one.
After criss-crossing the country over the xmas period i can tell you this charger is working fine for me. Even when using all the gps, bluetooth, screen on, etc that i can muster (while still driving straight!) the phone gains power and would often come out the car more charged than when it went in. The charger has one annoyance - they've built in a blue led to the charger which is lit up the whole time the thing is plugged in and thats kind of distracting, i don't really know what they were thinking with that, but otherwise i've no complaints.