5. Mr Simpson was called as an expert witness on the subject of ball bearings and castors. In a written report which was tendered in evidence, Mr Simpson said that it was "normal within Industry generally to regularly maintain wheels, castors and the like". In oral evidence, he said that the problem of sticking or jamming castors was "well known throughout the industry generally", and that it was easily overcome. He was not questioned as to "the industry" of which he spoke and it is possible that he was speaking of industry generally(1). He said the most common cause of the problem was threads of dust, but acknowledged that rust could also cause jamming. His evidence was that the problem could be avoided by the application of oil from an oil can, or by the application of a penetrating lubricant from an aerosol can, to the swivel and axle of the castors. The simple solution, he said, was "(a) squirt of WD40 from a(n) aerosol can, using the long thin plastic pipe that you get with them". That was so, he said, because it had the double effect of lubricating the rolling or sliding surface and of blowing out dust and dirt which is also partly washed out by the spray. There was no evidence whether the castors in question were made of metal or polycarbonate, but the clear inference from Mr Simpson's evidence was that, whatever the material, the regular application of a penetrating lubricant in the manner described would prevent jamming.