http://www.networkrail.co.uk/Use-of-standard-gauges.pdf This seems to be a reasonably introduction to gauging on the mainline, especially fig 2 on page 8. It seems that the below solebar level, under 1m above the railhead is the most problematic area, which is of course where outside cylinders come. *If* I've done my reading and sums right the standard for this area is now 2700mm (~ 8'10 3/16"?) and the paper suggests its better to work to 2650mm (~ 8'8 1/4" ?) So its little wonder Halls and Manors are having gauging troubles. Thanks andalfi for the typo spot!