Paul Sykes is David Cameron’s worst nightmare: a Ukip supporter with a pile of cash, and ready to spend it. By his own reckoning, this 70-year-old Yorkshire businessman has donated around £6 million to campaigns designed to keep Britain from the clutches of Europe. Now, after a lengthy period avoiding the political limelight, he is again about to dig deep into his own pocket to finance one more effort to sever the UK’s 40-year tie-up with the Continent.