If you're going to keep your player in your car most of the time, don't use an inverter. Your best bet is a DC-DC converter, such as the one at http://scott.kincaid.net/. Yeah, it's not a UK site, but the unit is tiny and it'll still cost less than an inverter, even with shipping. Just a suggestion.
I got my inverter from an Canadian company called Statpower http://www.statpower.com
They wouldn't sell to me directly so a friend from New Jersey bought it for me and brought it over to the UK when she came to see me!!!!
I'm now building one for a few friends, and the power unit I will be using is the same as above http://scott.kincaid.net Cos I can't think of anywhere cheaper than £27!!