Cheers.
It was expensive and some people will no doubt be shocked at the cost, but I look on it as an investment, as the car didn't have any sat nav when I brought it and considering the RNS 510 is now a £1800 option (not available when I brought the car), the £1000 I paid off ebay seems like good value to me! The unit I got was brand new and 6 months later is fully working and does exactly what I wanted it to.
The only problem is the postcode entry, which I'm told is being fixed in the new firmware release 1022/2022.