David Cameron is beginning marathon EU budget negotiations with a series of private talks in Brussels before joining a summit which could last into the weekend.
Other EU leaders will be ushered in throughout the day to see the two presidents as part of an unusual pre-summit effort to avoid deadlock when the summit gets under way over dinner. Only one item is on the agenda - the size of the EU's budget for 2014-2020.
Mr Cameron wants at least a real-terms freeze in EU spending, if not actual cuts, in line with austerity savings being made in most EU national treasuries.
But he will insist that Britain's EU budget rebate - worth nearly £3 billion a year off the UK's annual contributions to Brussels - is not up for trimming back as part of any deal.
As Mr Cameron prepared for his breakfast encounter, a pre-summit compromise was already on offer - a seven-year budget "envelope" of 940 billion euros (£756 billion) for 2014-2020 - a cut of nearly 5 billion euros (£3.8 billion) compared with the 2007-2013 ceiling.
The move was seen in Downing Street as a move in the right direction - although the "cut" is in a spending ceiling which officials say has not been reached. It is also above the 886 billon euros (£712 billion) originally pitched by the Treasury as in line with the real-terms freeze Mr Cameron wants.
But in the complex world of EU budget economics, with financial "commitments" different from "payments", a range of calculation options, rebates for some countries, and contributor and beneficiary member states, Mr Cameron and his colleagues have plenty of scope for claiming summit success.
The Prime Minister's allies for budget belt-tightening including Sweden and the Netherlands, who have both demanded hefty financial cuts. Germany, France, Finland and Austria want to freeze the maximum Brussels can draw from member states every year - leaving plenty of scope to argue over the actual spending figures within the ceiling.
A deal could take days to secure. "Mr Van Rompuy will keep the leaders talking as long as it looks likely that there can be agreement" said one official. "But he will not want to keep them at the negotiating table all weekend if the result is going to be a failure. There is the credibility of the EU to consider - better to end early and return another day than end in complete disarray."