2d. seize power in cy(PRUS U)sing backup. Usurp is in the clue backwards and means seize power.
18a. Anagram of 'cove on stage'. Steve Coogan plays a character called Alan Partridge.
Thanks torvic again and Mick M - the TV programme clues this week just happen to be programmes that I don't watch. One of the other clues was associated with The Simpsons.