By Alastair Macdonald

BRUSSELS (Reuters) --  David Cameron will have an expectant but not especially patient, or even attentive, audience when he meets fellow  European Union leaders at a summit in  Brussels at the end of this week.

"Waiting for David" has become something of a catchphrase among diplomats and officials, who say the British prime minister is frustrating them by not yet detailing his demands for new EU legislation ahead of the membership referendum he plans to hold.

British officials insist it is vital to prepare the ground and take time to find reforms to benefit all 28 member states.

But as 'In' and 'Out' campaigns gear up, a fear of partisan leaks in Britain's boisterous media means there has been no circulation so far of the kind of written draft proposals that are the lifeblood of the legalistic, and leaky, EU bureaucracy.

"They need leaks like a hole in the head," said one person close to talks in Brussels. "This is an incredibly delicate operation for them domestically."

The agenda for Thursday and Friday includes a briefing by summit chair  Donald Tusk on the "state of play" with Britain and how he will conduct negotiations before a vote expected between early next summer and an end-2017 deadline.

But few expect to hear much more during a meeting that will be dominated by the Syrian refugee crisis, including relations with  Turkey and  Russia, and by proposals to bolster the euro zone following the latest chapter of the Greek debt crisis.

"David Cameron is very discreet -- and I can understand why," a senior EU official said after Cameron's last visit two weeks ago, when he met Tusk, president of the  European Council.

British voters' perceptions of whether Cameron has succeeded in his negotiations will be crucial -- making it important to manage appearances of what he will ask for and what he achieves. And it is too early to assess what other leaders will accept.


British sources say that "technical talks" held since Cameron's reelection in May have merely been to set up a framework for negotiations and clarify broad areas where Britain wants change: relations between  London and the euro; a more efficient, less federal EU; and, critically, curbs on immigration by EU citizens.

Now negotiators are preparing to move beyond that stage to prepare for a first discussion of concrete reforms, including possibly changes to EU treaties, when leaders meet again in mid-December. "Things are warming up," one told Reuters.

But with Cameron in command of the vote timing, and refusing to prejudge negotiations by backing an 'In' vote, one minister played down expectations during last week's annual Conservative conference even of there being much progress by December.

"If you want a deal, you have to write things down," a second senior EU official told Reuters, voicing a measure of frustration that  London had yet to focus its demands. "And no one wants to write things down because everything leaks."

"Nothing's been written down," said another EU source. "Once you write it down, it'll leak. So the big question is: What do you write down, who does that and when?"

Among officials in Tusk's  European Council, the forum for national governments, and the executive  European Commission run by  Jean-Claude Juncker, with whom Cameron will lunch on Thursday, there is an insistence on a will to negotiate and a confidence about coming up with a "fair deal" for Britain and the EU.

But on both sides of the table there is a growing awareness of the unpredictability of referendums and of the scope for mutual misunderstandings before 2017, a year that will also feature elections in big EU powers France and Germany.

One senior EU official, noting Britain's combative politics, said Cameron would need to show he had "won one big fight" in Brussels to convince voters to back any new deal. But the official added: "I'm not sure the Europeans are ready for that."

Another EU source warned that, if Cameron reprised tactics of going home from Brussels declaring "I won", and failed to swap confrontation for consensus, then other leaders could block his demands, however much they want to keep Europe's second biggest economy in the EU.

"If he wants headlines saying the UK humiliated France, he will fail," the official said. "This is not rugby."

"Bandwidth" Issue

British officials say Cameron and his lead negotiator, finance minister George Osborne, have learned lessons from a bruising experience four years ago when Cameron surprised and infuriated EU peers by imposing a rare summit veto.

Both have spent recent months cultivating ties with other governments. And London is confident that EU leaders do not want to see the bloc badly damaged by the Britons voting to leave.

Nonetheless, British officials also acknowledge a "bandwidth issue", aware that EU leaders' time and appetite for appeasing Cameron is limited as migrants,  Russia Greece and the euro zone economy all seem to threaten the bloc's very existence.

"The EU is limping from one crisis to another and there is a risk of the British issue being squeezed," one said.

This week's summit briefing from Tusk, with possible input from Cameron, fresh from a weekend spent in part with German Chancellor Angela Merkel, is intended at least to reassure other leaders they are being kept informed, another official said.

However, the most frequently asked question he may hear from some fairly exasperated counterparts is "What do you want?".

"We still just don't know what he's really asking for," one senior diplomat said, echoing a commonplace in the EU capital.

"Does he even know himself?" asked another envoy, from a government seen as one of  London's closest allies in the bloc.

"Frankly, he's been very silly. No one wants to see the  United Kingdom leave. But it's up to Cameron to sort this out."

An EU official familiar with an area of great interest in  London concerning migration within the EU said staff had not seen proposals from Britain on how it might limit benefits for EU workers.

"We want to work with them, help them. But this can't just be about more opt-outs," the official said. "He called this referendum. So it's up to him to come up with ideas to get himself out of this situation."