Welocome along.
To be blunt, if we can afford £20 odd K cars, why do we worry about saving £100 on a new radio for it?
If you hold out, you will find an as new, genuine, original RCD510 on ebay for less than £250. I initially got stung by buying a radio outside of ebay that never turned up. It took me ages to get my money back. I then looked for another one by refining my search to my local area. Within a few days, one turned up less than 20 miles away. It had been taken from a brand new Eos when they upgraded to a Kenwood Nav unit. I managed to do a deal with them outside of ebay so they saved their fees, and I didn't have to hand over the cash untill I had the radio in my hands.