It is fair to say 2015 was a momentous political year and 2016 is almost certinly likely to be one too.

David Cameron now has his sights clearly set on holding a referendum on the UK’s membership of the European Union in the summer.

His charm offensive in Europe has charmed some and offended others.

Essay: Michael Settle on what will happen if we leave the EU  

There has been talk of fairness and flexibility on all sides, a desire to keep Britain – the EU’s second largest economy - in the Brussels bloc but the Prime Minister’s proposed four-year ban on welfare for migrants has stoked deep opposition.

Our central and eastern European partners in particular insist this would breach the fundamentals of not discriminating between EU citizens and maintaining the free movement of labour; and up with Mr Cameron’s plan they will not put.

The Prime Minister is keen to cut a deal at the meeting of EU leaders in February. Given the profound lack of any agreement over a Brussels dinner last month, one cannot doubt his optimism but the reality is likely to lead to a classic fudge, which Mr Cameron will hail as a Thatcher-like triumph but his critics will see as a humiliating flop. And so the battle lines will be drawn.

While the campaign to leave has set the early pace, it is difficult to see beyond the arguments for the UK staying in the EU, albeit in a slightly altered and more flexible form.

With a marketplace on our doorstep of 500 million people, businesses, with the UK in the EU, have to abide by just one set of rules; Brexit could mean a whole set of new ones, including punitive tariffs.

The UK could decide to abide by certain EU regulations but, once it leaves, it will not be able to have any say over them and could be put at a serious economic disadvantage.

The US and China invest in Britain partly because of its key place within the EU marketplace. Brexit could make the UK a less attractive place to invest; business and influence would be lost in an increasingly competitive world.

Of course, while the arguments over jobs and economic factors will be batted back and forth, for some the decision will come down to a question of identity.

Being or feeling European is not synonymous with being part of the EU but for many the two are strongly interlinked.

A vote to stay in the EU would, hopefully, settle the issue once and for all; at least, that is, for another generation.

But a vote to leave the club of 28 would have profound consequences not just in terms of an EU perspective but elsewhere too.

Brexit would, given the magnitude of the issue, almost certainly result in Mr Cameron’s resignation and spark a potentially nasty fight over the Conservative crown.

And there is, of course, the implications such a vote would have here in Scotland.

If the UK votes to leave the EU while Scots vote to stay, the pressure on First Minister Nicola Sturgeon to call a second independence poll would be irresistible.

So what might be at stake in June or July is not just the future of one union but two.