Damien

The Government almost certainly has polling on this. Governments poll people all the time so do the parties. The thing is they will be less interested in the headline result than they are the other questions that accompany the poll which will be used to direct their message/campaign. The revelation of which will be a hinderance to the campaign as it let's your rivals know what you're thinking as well as providing, for free, the results you got.

It's very likely that the Government polled on the question of a Currency Union before making their intervention for example.

The secret poll is just spin by the Yes campaign to fuel their sense of paranoia. It sounds dodgy but happens all the time. Governments poll on a lot of things.