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The only explanation for the title that I can see is, I admit, rather lame, i.e. the eight unclued lights are all (UK) towns, albeit 'shanty towns' in the sense that they are 'ramshackle' and have to be 'rebuilt'. Anything else the eight have in common I fail to see. This explanation is also compatible with the diagonal town being simply the biggest town (in the UK). Or do they really have something else in common?