My day job is to be the UK Permanent Representative to the UN, covering the Security Council, the General Assembly and the rest of the UN family. It is my night job too.
Tonight I want to talk about what this diplomat actually does, to outline some of the issues which I confront in my daily work, and try to explain what we in the UN are doing.
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In the last three weeks, I have spent much time trying to limit Iran's nuclear ambitions.
Problem: 18 years of clandestine nuclear activity only admitted by Iran in 2003 when confronted with incontrovertible evidence.
At first, Iran claimed domestic development of its enrichment programme, but subsequently admitted collaboration with the AQ Khan network.
Iran has not explained the rationale for the heavy water research reactor for which there is no convincing civilian use. But such a reactor would be ideally suited to a programme to produce plutonium for nuclear weapons.
In a recent report the IAEA said Iran has in its possession a document on the casting and machining of uranium hemispheres. Only credible application for such item is in a nuclear weapon programme.
No economic rationale to Iran's nuclear programme. Iran has claimed that the programme would support its nuclear reactors, but it currently has no functioning power reactors needing fuel.
Iran makes much of its right under the NPT to develop peaceful nuclear technology. But Iran also has obligations.
In 2003 Iran was found by the agency to be in breach of its NPT safeguards' agreement.
In successive reports, the Director General said that it was essential that Iran should answer a series of questions as honestly and transparently. Three years of intensive inspections have not clarified all outstanding questions, nor been able to confirm that there are no undeclared material or activities in Iran.
Iran protests that research and development is legitimate and not really enrichment. But R and D would allow Iran to master technology. There is no way then to cap the technology. Iran could easily enrich natural uranium to weapons grade material and hence produce a bomb.
There is no real difference between R and D, a pilot programme for enrichment, or enrichment itself. One centrifuge perfected can become 1400 perfected. Enrichment to 4% can become enrichment to 80%. Net result sufficient nuclear material to produce a bomb.
Once technology is mastered, it can be replicated at covert facilities away from the safeguards of the IAEA.
There is an absolute lack of confidence in Iran's objectives for its nuclear programme and a real proliferation risk which the international community cannot afford to take.
So what is the UN role and why is action vital?
Iran needs to build confidence about its nuclear intentions. Europe takes the view that until confidence is restored, Iran should refrain from the most sensitive fuel cycle activities.
Despite repeated demands by the IAEA Board of Governors to suspend all enrichment-related activities, Iran has flagrantly proceeded with its nuclear fuel cycle research. As negotiations continue, Iran just works towards perfecting the technologies that could provide the basis for nuclear weapons production.
In February the IAEA Board asked its Director General to report the matter to the Security Council.
On 29 March, the Council adopted by consensus a statement calling on Iran to take the steps required by the IAEA Board, and requested the Director General of the Agency to report back in 30 days on the process of Iranian compliance.
The process is reversible. The ball is in Tehran's court.
But if there is no progress on compliance, then the Council will return to the issue. The Security Council has the authority to impose mandatory obligations on all UN Member States.
So the Security Council reinforces the policies and actions of the IAEA, whose primary responsibility it is to negotiate with Iran.
But bottom line, it is the Security Council which has responsibility for the maintenance of international peace and security.
So the choice is for Iran. It can suspend nuclear activity and resume negotiations, and the Security Council involvement will end. It would open the way to what the Europeans favour, a new stronger relationship between Iran and the EU.
But if Iran ignores what is available, and instead defies the international community, then it will precipitate a crisis and an escalation.
On these key strategic principles, all Permanent Members of the Security Council agree: that proliferation poses a threat to international peace and security; that such threats fall primarily to the Security Council; and that Iran should not be allowed to develop nuclear weapons.
A key issue for us all, and one where the Security Council is discharging its responsibilities.
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