if its a steel frame (and I'm guessing the Mercian is steel) then you can get the back end respaced to 120mm for a tenner - a visit to Mario Vaz in Catford will sort it (PM me if you want the number) or indeed any framebuilder will do it. He can also graft track ends onto a regualr road frame for £50 but you'll need a respray after (but he does resprays too).
130mm hubs often require longer bottom brackets - better to stick to 120mm. The on-one hub needs a bottom bracket of 109mm for a perfect chainline.