Complex sentence: Only one of the six strong class has made it into preservation and it is the late Viscount Garnock we have to thank because on thier withdrawal from service he bought No.61994 "The Great Marquess" from British Railways and had the engine overhauled to working order.
Split into simpler sentences: Only one of the six strong class has made it into preservation and it is the late Viscount Garnock we have to thank. Because on there withdrawal from service the Viscount Garnock promptly bought No.61994 "The Great Marquess" from British Railways and had the engine overhauled to working order.