I never reckoned higher than 50:50. But the negotiating positions are really not on different planets, in many respects they are well aligned. But there are genuine divides of political and economic philosophy...
The current UK government believe we are at least the equal and probably the superior of the EU as one sovereign state versus what is regarded as an unstable and dubious 27. We should therefore set the terms. The EU looks at market power and says they should set the terms.
For at least some in the Conservative Party and wider Brexiteer movement our departure is all about trying to break the EU. For some in the EU any deal with the UK must keep us in their regulatory orbit.
The economic philosophy is now radically different. The EU seeks regional integration through a multitude of continent wide regulatory structures. The UK used to be a proponent of this but has changed completely to a more 18th or 19th century notion of pure global free trade.
The EU believe it is important to tackle tariffs and non-tariff barriers to trade. David Frost's latest speech suggested that non-tariff barriers are exaggerated, and the UK only appears to be aiming at removing remaining tariffs as the main goal of trade talks. 
Trade negotiators can overcome issues of detail. Matters of philosophy are more difficult to overcome, not least as neither side is particularly united (business in the UK don't recognise government economics, some in the EU do want to punish the UK).
So 50:50 because although the philosophy is important, so is the desire of politicians for a deal they can wave to show they won. Not to mention that the UK government is trying to take on an awful lot of opponents at the same time. /end
PS This is important - the UK is not terribly well united right now
TL:DR - the worst allegations of both UK and EU about the other are true to an limited extent, however much that is denied. If those voices are dominant you can't have a deal. But if you ignore their presence you also won't get a deal.
