To add to the confusion, the BO105 is also used by British Transport Police and the contract includes Chiltern.
If it was red, green and white and driven by a woolly chap who was singing it may have been the Veritair BO105 out of Cardiff on a railway job.
Do you have a railway [train set in loft does not count]?
If it was normal colours [blue/yellow] the plot thickens.