I think it was a ban on European migrants from sending benefits and particularly child benefit (some money you get from the state if you have kids) back to thier home country or something like that.

I remember Poland stopped us from getting it (supposedly).

The french didn't want us to exempt from financial regulation that was primarily targeted at the euro even though we use the pound.

Various nonsense, that people at the time felt strongly about (keeping the pound, not giving benefits and a council house to people as soon as they arrived, or some such exaggeration)