A coalition dominated by the Conservatives will ensure that a referendum bill is in the first Queen’s Speech of the new parliament. Since every new intake of Conservative MPs tends to be more Eurosceptic than the last, it will be difficult for a Conservative Prime Minister to push the reform rather renegotiation route; however, renegotiation would mean treaty change, and there is no appetite for this in Berlin, or many other capitals.
Even in the event of a Labour victory, the risk of Brexit will not have disappeared completely. The new government may hold only a tentative grip on power, only to lose it in the face of growing unpopularity, and go into another election against an even more Eurosceptic conservative leader.
Unlike many other European states, the UK has no particular historic argument for being a member of the EU beyond the economic sphere. The EU is not viewed as a source of democratic strength (Spain), or of better governance (Italy), or a good way to tame German economic power (France). This suggests any referendum debate will be dominated by the difficult to answer question of whether Brexit would be a net benefit to the economy.
Much will depend on the terms which the UK is able to agree with its former partners. Even if the electorate votes to stay in, there is a danger that, as with the recent Scottish referendum, one vote does not settle the issue.