let's not forget the part UKIP had in forcing Camerons hand, they did well in the European election 2014
Not sure about UKIP forcing his hand, but I do think that the referendum promise was calculated to avoid losing votes to UKIP. Of course, they didn't think they could win a majority, so didn't expect to have to deliver on the promise. This has now blown up in Cameron's face as it looks inevitable to me that immigration fears will dominate the debate, resulting in an exit vote. This will be followed by another referendum in Scotland and a break-up of the Union. Good job Dave.