as an adjunct to Sol's excellent tip.......Clover House are planning to close down, in its present form....so I suggest hurrying-up?
just in case?
Also note, Clover house products will be scaled for 3.5mm HO......as well as others, including american O scale, which is slighly different to UK O scale...[they use frequently, a ratio of 1/4 inch to foot].....
David, are you looking for 3.5mm or 4mm scale sleepers?