Need to charge up my battery after leaving the lights on overnight. According to the manual the battery needs to be disconnected before connecting a smart charger. Will I lose my pre-set radio functions on disconnection or can the charger be connected to the positive terminal and an earth point on the car without any damage to the sensor on the negative terminal?

You can connect the charger whilst the battery terminals ares still connected to the battery.

Just ensure that you disconnect and reconnect (negative last on and first off) the charger switched off first otherwise this can create a spark and ignite the Hydrogen gases that build up around the battery when charging.

Also ensure that the area is well ventilated to avoid gas build up