Yes, just don't accept bad work. If everyone takes that view, those who fail to do things to an acceptable standard will soon go out of business. Any problems, i.e. if he refuses to put matters right, the Citizen's Advice are the first people to ask about your legal rights, or just get the form from the Small Claims Court and put that process into motion. Most people who are in the wrong cave in and pay up at that point, because they will often lose more money taking time out to attend court than to fix the problem. A few will be foolish, and defend their cases, but consumer protection law is very much in favour of you.
As to your question, the final drive ratio can be changed, somewhat awkwardly, with the engine in situ, but changing just the speedo drive is somewhat easier, although the only time I needed to do both was when a replacement engine and box turned out to be from, presumably, a Metro. It accelerated rather briskly, but not to an impressive speed, as it was geared for much larger wheels, so to avoid lifting it out again, because I was just fed up with doing that, I did it the hard way, knowing that in the early days certain rally teams had managed to change diffs by the roadside. It is definitely a job for those who like working in confined spaces. Not sure if I would do it that way again. It perhaps saved an hour or so, but sometimes the less stressful way is best, even if it takes longer.
