Mum is still engaged on her quest of trying to prove she became a naturalised UK citizen in 1969 (the eventual goal being to prove she _isn't_ a South African citizen any longer and therefore doesn't need an SA passport to enter that country). She has now heard back from the Immigration Authority saying "yes, your naturalisation number was X, the date concerned was Y, but we no longer have a copy of your certificate because it was so long ago; you need to contact the National Archives at Kew".
rotwang has just hypothesised that Presumably they need to find someone who can still transcribe cuneiform, some of the right sort of clay, etc.?. I am inclined to agree. Did they issue certificates as cave paintings?