I think the PM’s main concern was the avalanche of financial regulation that was likely to spew out of the EU as it struggled to contain the Euro crisis. Mr Cameron wanted an agreement that the European Banking Authority would remain in London, protection for US financial institutions based in London that do not trade with the rest of Europe, and an agreement that any changes - including a financial transactions tax - would require the unanimous backing of all EU members.
He got none of those. There is no doubt that with the UK’s dependence upon the finance sector for about 10% of its GDP, such regulation would have a disproportionately high impact on the UK. There is much talk that the PM is only concerned with protecting his mates in the City of London. But the finance sector employs more than a million people right across the UK and generates 11% of the nation's tax revenue. The Prime Minister would have been negligent if he allowed treaty changes which would jeopardise that industry.