Eurosceptic Conservative backbenchers in the UK want to use the summit tomorrow to negotiate the return of powers from the EU to the UK. It's not clear what powers they want to return to Westminster, but it's likely that the area they're interested in is social policy. This area covers things like the 35 hour week, maternity leave, and holidays. The problem is that Britain isn't in a good negotiating position.
First of all, the official position of the British government is that they want the Eurozone to have some level of fiscal union so that the Euro survives and the British economy is protected by meltdown. It would be a bit strange if the UK government suddenly switched from cheer leading greater integration to threatening to block it so other treaty areas could be opened up. Even if there weren't any other problems with the renegotiation position, this would leave the renegotiation position without credibility. After all, which does Britain need more at the moment: a stable Eurozone or a full treaty renegotiation?
Second, any treaty change will be designed to only affect the Eurozone so the UK's referendum law won't be triggered. So no powers will move from the UK to the EU. But at the same time Britain, as stated by Cameron, wants safeguards that ensure that the Eurozone "Outs" are protected from the growing integration (and potential power) of the "Ins". So Britain (and the other non-Eurozone countries) want something from these negotiations, but they will not be offering anything on the integration side. Whie Germany and other Eurozone countries (like Ireland) support a treaty change for the 27, it has already been signalled that the Eurozone 17 could go on ahead with a treaty outside the EU if necessary. When you're in a position of asking for safeguards but not exchanging anything in return, with the possibility that your negotiating partners can ignore you altogether, it's not a very strong negotiating position.
Third, the UK underestimates how controversial returning social policy will be for the other EU Member States. Social policy seems to be portrayed like it is a small add-on to the internal market, but for other Member States this protects their social policies and welfare states from the opening up of their markets. Why should the special access the internal market provides be given to the UK if they are going to engage in race-to-the-bottom social practices that would harm their welfare states? To put it in terms of the UK's human rights debate: there are rights and responsibilities, and the UK is increasingly seen as wanting all of the rights, but none of the responsibilities.
So for Britain to successfully negotiate a return of social policy powers to London, they need allies (which they would loose from obstructing a treaty change that both Germany and other Outs like Poland want), be in a position to offer something in return (to reassure other Member States that they won't start a race to the bottom), and a credible negotiating position (i.e. not being dependent on the treaty being passed and the goodwill of other Member States to be a part of the negotiations). I can't imagine that the Conservative backbenches aren't already aware of this (I mean, if they aren't I'd like to see their negotiations!), so it could be a way of trying to force David Cameron into backing an In-or-Out referendum.
Cameron might have more luck with negotiating safeguards on financial services, though the 17 can still threaten to go on ahead without Britain. He'll have to hope that whatever safeguards he gets will satisfy the Tories back home.