My keyboard connected smoothly. However the mouse was more problematic. Vista could see the mouse but seemed to wait for a security key or something that never came.
While trying the earlier method, going to Control Panel -> Mouse -> Bluetooth I noticed that the device was visible and identified as a Logitech mouse. Went to the Logitech support page and looked for Bluetooth mice in the hope that the Dell mouse was really a Logitech product.
Downloaded and installed the driver for the Logitech V470 and now both keyboard and mouse work okay.