IF body language is any measure, then yesterday’s Brexit "breakthrough" seemed to go down well with both Prime Minister Theresa May and European Commission president Jean-Claude Juncker. There was a palpable sense of relief on the faces of both leaders as they emerged from talks following a week of high drama and something of a political nightmare for Mrs May.

The signing off of a 15-page “progress report,” will now allow EU negotiators to recommend opening a second phase of talks on post-Brexit relations. On the face of if what has been agreed seems significant enough. There are guarantees that there will be “no hard border” between Northern Ireland and the Republic maintaining the “constitutional and economic integrity of the United Kingdom.”

EU citizens, too, came out fairly well with those living in the UK and vice versa having their rights to live, work and study protected.

And of course there was the question of the so-called Brexit “divorce bill,” which the UK Government now says will amount to between £35bn and £39bn, though there does seem a lack of precision here.

Indeed it’s this lack of precision in a number of areas that still gives cause for nervousness with a real suspicion that the devil continues to lie in the detail.

Perhaps nowhere is this more apparent than on the issue of Ireland and border controls.

As agreed yesterday it commits both sides to an open border between Northern Ireland and the Republic, and “no new regulatory barriers” between Northern Ireland and the rest of the UK. That said, it also commits to the UK leaving the EU’s single market and customs union. These respective positions are in themselves something of a contradiction.

For if Britain does become a “third country,” that is trading with the EU as other non-EU countries outside the single market and the customs union do, then border controls will be necessary.

Arlene Foster, the DUP leader, underlined these concerns making clear yesterday that she still had reservations about the final wording of the deal on the border.

Indeed the interpretation of so much agreed remains up for grabs and there is a sense that the can has simply been kicked down the road on the most contentious of issues like Ireland.

Let’s not forget that this, the "easy" part, has taken a full year to agree and we now have less than a year to negotiate a substantially harder part.

As Michel Barnier, the EU's chief Brexit negotiator, pointed out, the UK now faces the prospect of a Canada-style trade deal with the bloc, an accord that is primarily focused on goods rather than services and falls far short of Britain’s current full access to the single market.

The bottom line here is that the overall terms arrived at yesterday are closer to the opening demands made by the EU, including the size of Britain’s exit bill, the breadth of protected citizen rights and commitments made regarding the Northern Ireland border.

As Scotland’s First Minister Nicola Sturgeon said of yesterday’s last-minute deal, this is a welcome step forward in the negotiations, but the next trade arrangement stage will be significantly tougher. There remain difficult and uncertain times ahead.