The K550 seems to have the power/connectivity socket in the same place as the C902, so the new adapter might fit it anyway, but £55-£60 is a lot to risk. And even if it does physically fit, it doesn't mean the phone will work with the system in the car.
BTW, the C902 camera picture quality is not very good (even with twice the resolution of the K800), and the LED flash is nowhere near as good as the xenon flash on the old K800i. If I had the choice, I'd swap my C902 for a C905.
The snap-in adapter for the C902 is part number 84212151737.
BTW, the part number for the Nokia 6230 should be 84216973983 - don't know what the extra 2 digits on your part number are, maybe a version/date?
I only use the adapter because I can't charge it up in-car otherwise, and some of my drives with the phone's bluetooth on can see my phone die after a few hours (including calls of course). At least with the K800 adapter you got an antenna extension to give you a better signal, don't know if that's possible on the C902/C905?