@ScottishM: To start with, please restart your phone and check that the phone's software is up to date by going to Settings > Software Update. Furthermore please go to Settings > Connections > Bluetooth and ensure that it is enabled. Then, make your Bluetooth device visible by following the manufacturers instructions, select it from the list of available devices, enter a PIN if prompted and you should be good to go.
If the issue persists then head into Settings > Apps > Tap the 3 lines with the downward facing arrow to the right of 'Your Apps' > Show System Apps > Bluetooth > Storage > Clear cache / data. If this doesn't help, go into your Bluetooth settings, select the device you are having problems with and then tap "Forget" or “Unpair” from that device and then restart your phone. If the problem persists then head to the Samsung Members app > Get Help > Interactive Checks > Bluetooth, and follow the on screen instructions. Does this return any errors?