Hi David, don't know if he ever came back, so will try to assist meantime. From underneath the sink, the order of components are: a rubber seal, then a horseshoe (or C shaped) metal fitting which is used to clamp the monobloc to the sink, and then a brass nut. Because access is often VERY restricted under there, the ideal tool to tighten this nut is a long tubular spanner. Cheap to buy, but specific for use. Making sure the tap is straight it only needs a quarter or half turn to do the job, but as mentioned access may be limited/awkward to crawl under. Your plumber was in too much of a hurry. Poor show, for what they charge! Hope it helps. Paul's DIY Services, Bearsden Glasgow.
Paul
January 2014