This is for two tyres.  These tyres have an unstretched o/d of 9mm, i/d 8mm, width 1.5mm.  These measurements are approximate. They fit a range of Hornby diesels and DMU's including Class 55, 73, 101, 156.  I am pretty sure they also fit the Hornby Dublo Class 20 3 rail (D8000).

Some of the service sheets to reference are: 292, 296, 301, 303, 320,  323, 334.