IF YOU find yourself spending the night in your local jail, or trying to collect your ,council's tax, or riding shotgun in a squad ;car, then you will have been caught up by the enthusiasm of Julia Middleton. It is hard to avoid. She rightly thinks that the people who make things happen in most cities are at cross-purposes — not knowing each other, and certainly not understand- ing each other's work. So she raised £500,000 from eight companies (like Brit- ish Petroleum, National Westminster and Grand Metropolitan) and set up Common Purpose, which has Brandon Gough of Coopers & Lybrand for its suitably forceful chairman. It runs programmes, for 30-odd people at a time, picked from business, local government and services: 'There has to be someone from the police, from the new hospital trusts, a very senior nurse anyone in a decision-making position — you don't want a group of people who are going to agree with each other.' They disagree in business-school exercises case studies, choices, negotiations, cross- examinations. They have days (with eight o'clock starts) on such topics as crime and justice, which might take in the Inspector of Prisons and a harassed inspector in a front-line police station. From Newcastle and Coventry, the programmes have spread as far south as Islington, which had local-government jokes to live down. (What does an Islington councillor call his pet zebra? Spot.) It is a good idea, Julia Middleton says with emphasis, and the timing is absolutely right. Events make the case for her.