Thank you all for all your help. The 'dish out' on the rear wheel after the job wasn't sufficient and the bike mechanic had taken a shortcut to align the rim with the chain stays by fitting the axle skewed. Now I have a properly aligned rear axle with the rim exactly equidistant in the chain stays. However, the largest gear out of 5 (22 teeth) causes the chain to slip off the singel chainwheel (the chain falls on the inner side - between the frame and chainwheel).