Hi - my 900bt has had a problem from day 1 whereby it does not remember radio presets or sound settings. Every time I start the car, the setting are wiped and its like a new install (it does remember maps etc).

How do I fix this?

Did you connect the wiring harness' main power to the car's "always on" power, or to the "accessory" power (which goes away when you turn the key to off)?  What you're describing is the same result you'd get if your car battery died or was disconnected, which removes the "always on" power.

How do I fix this?

well your unit is either broke and needs to be sent in to be fixed, or there is a problem with your install like having the ACC and constant wires hooked up backwards.

what kind of car is this installed in?(this should always be listed when asking for help with a problem like this as car specific can make a huge difference in where to start troubleshooting)  the reason i ask is some cars use other cars harnesses but sometimes the pin out is different

for example: certian model ford focuses and a few Mercedes use a Volkswagen wire harness, everything is the same EXCEPT for the red and yellow wires are reversed (so when installing the radio, the radios red wire goes to the harnesses yellow wire, and the radios yellow wire goes to the harnesses red wire)

regardless, to figure this problem out, you will need a meter, and will need to check to make sure that the radios yellow wire has 12v 100% of the time, and that the radio's red wire only has 12v when the key is turned on and has no voltage when the key is turned off. if your car does meter correctly, then there is an internal problem with your unit.