Thank you kindly. I have now taken the neck off and refitted it, ensuring the bolts are nice and tight. I also raised the saddle height by two full turns of the saddle screws to be certain that the string 'upwards' of the fifth fret was not being obstructed by a high fret - plus checked clearance across the string whilst being fretted and plucked. Unfortunately, the dead spot is still there, so will now try a set of Boomers as above (current strings are indeed, Swing Bass), and if that doesn't work, I have managed to find a friend with the famous 'Fatfinger'. Thanks to all who contributed thus far. I'll keep you posted as my efforts continue.