Nigel Farage, the leader of the United Kingdom Independence Party (Ukip), has called the forthcoming by-election in Rochester and Strood “the most important for 30 years”. He has a point. The then Conservative candidate, Mark Reckless, won this Kent parliamentary constituency by nearly 10,000 votes in 2010, and the by-election arises from his decision to resign the Tory whip and join Ukip. Like fellow defector Douglas Carswell, who recently won the by-election in Clacton in similar circumstances, he was under no obligation to resign and stand again but wanted to strike a blow at David Cameron. This is one of the most serious challenges a Conservative prime minister has faced since the rebellion that brought down Margaret Thatcher in 1990.Mr Cameron’s main line of defence